Nope. Everything looks normal (at least to me).
ffmpeg output:
$ ffmpeg -f concat -safe 0 -i <(for i in ./title0[678].mkv; do echo "file '${PWD}/${i}'"; done) -map 0 -c:v copy -c:a copy titleall.mkv
ffmpeg version N-46247-g017a68e8f Copyright (c) 2000-2017 the FFmpeg developers
built with Apple LLVM version 8.1.0 (clang-802.0.42)
configuration: --prefix=/usr/local/Cellar/ffmpeg/HEAD-017a68e --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --disable-indev=qtkit --enable-libfdk-aac --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --disable-lzma --enable-nonfree --enable-vda
libavutil 55. 63.100 / 55. 63.100
libavcodec 57. 96.101 / 57. 96.101
libavformat 57. 72.101 / 57. 72.101
libavdevice 57. 7.100 / 57. 7.100
libavfilter 6. 90.100 / 6. 90.100
libavresample 3. 6. 0 / 3. 6. 0
libswscale 4. 7.101 / 4. 7.101
libswresample 2. 8.100 / 2. 8.100
libpostproc 54. 6.100 / 54. 6.100
Input #0, concat, from '/dev/fd/63':
Duration: N/A, start: 0.000000, bitrate: 4692 kb/s
Stream #0:0(eng): Video: mpeg2video (Main), yuv420p(tv, bt470bg, top first), 720x576 [SAR 16:15 DAR 4:3], 4500 kb/s, 25 fps, 25 tbr, 1k tbn, 1k tbc
Metadata:
BPS-eng : 4500000
DURATION-eng : 00:01:03.920000000
NUMBER_OF_FRAMES-eng: 1598
NUMBER_OF_BYTES-eng: 35955000
SOURCE_ID-eng : 0100E0
_STATISTICS_WRITING_APP-eng: MakeMKV v1.10.5 darwin(x86-release)
_STATISTICS_WRITING_DATE_UTC-eng: 2017-06-16 21:12:55
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES SOURCE_ID
Stream #0:1(eng): Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
Metadata:
title : Stereo
BPS-eng : 192000
DURATION-eng : 00:01:03.904000000
NUMBER_OF_FRAMES-eng: 1997
NUMBER_OF_BYTES-eng: 1533696
SOURCE_ID-eng : 0180BD
_STATISTICS_WRITING_APP-eng: MakeMKV v1.10.5 darwin(x86-release)
_STATISTICS_WRITING_DATE_UTC-eng: 2017-06-16 21:12:55
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES SOURCE_ID
Output #0, matroska, to 'titleall.mkv':
Metadata:
encoder : Lavf57.72.101
Stream #0:0(eng): Video: mpeg2video (Main) (mpg2 / 0x3267706D), yuv420p(tv, bt470bg, top first), 720x576 [SAR 16:15 DAR 4:3], q=2-31, 4500 kb/s, 25 fps, 25 tbr, 1k tbn, 1k tbc
Metadata:
BPS-eng : 4500000
DURATION-eng : 00:01:03.920000000
NUMBER_OF_FRAMES-eng: 1598
NUMBER_OF_BYTES-eng: 35955000
SOURCE_ID-eng : 0100E0
_STATISTICS_WRITING_APP-eng: MakeMKV v1.10.5 darwin(x86-release)
_STATISTICS_WRITING_DATE_UTC-eng: 2017-06-16 21:12:55
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES SOURCE_ID
Stream #0:1(eng): Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, stereo, fltp, 192 kb/s
Metadata:
title : Stereo
BPS-eng : 192000
DURATION-eng : 00:01:03.904000000
NUMBER_OF_FRAMES-eng: 1997
NUMBER_OF_BYTES-eng: 1533696
SOURCE_ID-eng : 0180BD
_STATISTICS_WRITING_APP-eng: MakeMKV v1.10.5 darwin(x86-release)
_STATISTICS_WRITING_DATE_UTC-eng: 2017-06-16 21:12:55
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES SOURCE_ID
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 5500 fps=0.0 q=-1.0 Lsize= 126132kB time=00:03:39.98 bitrate=4697.0kbits/s speed= 229x
video:120868kB audio:5156kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.085203%
MKVToolNix output:
--- Output of job 'Multiplexing to file "title06 (1).mkv" in directory "/Users/mn/work"' started on 2017-06-19 22:22:14 ---
mkvmerge v11.0.0 ('Alive') 64bit
'/Users/mn/work/title06.mkv': Using the demultiplexer for the format 'Matroska'.
'/Users/mn/work/title07.mkv': Using the demultiplexer for the format 'Matroska'.
'/Users/mn/work/title08.mkv': Using the demultiplexer for the format 'Matroska'.
'/Users/mn/work/title06.mkv' track 0: Using the output module for the format 'MPEG-1/2'.
'/Users/mn/work/title06.mkv' track 1: Using the output module for the format 'AC-3'.
'/Users/mn/work/title07.mkv' track 0: Using the output module for the format 'MPEG-1/2'.
'/Users/mn/work/title07.mkv' track 1: Using the output module for the format 'AC-3'.
'/Users/mn/work/title08.mkv' track 0: Using the output module for the format 'MPEG-1/2'.
'/Users/mn/work/title08.mkv' track 1: Using the output module for the format 'AC-3'.
The file '/Users/mn/work/title06 (1).mkv' has been opened for writing.
Appending track 1 from file no. 1 ('/Users/mn/work/title07.mkv') to track 1 from file no. 0 ('/Users/mn/work/title06.mkv').
Appending track 0 from file no. 1 ('/Users/mn/work/title07.mkv') to track 0 from file no. 0 ('/Users/mn/work/title06.mkv').
Appending track 0 from file no. 2 ('/Users/mn/work/title08.mkv') to track 0 from file no. 1 ('/Users/mn/work/title07.mkv').
Appending track 1 from file no. 2 ('/Users/mn/work/title08.mkv') to track 1 from file no. 1 ('/Users/mn/work/title07.mkv').
The cue entries (the index) are being written...
Multiplexing took 1 second.
ffprobe output of the MKVToolNIX concatenated file (with metadata removed):
$ ffprobe titleall_no_meta.mkv
ffprobe version N-46247-g017a68e8f Copyright (c) 2007-2017 the FFmpeg developers
built with Apple LLVM version 8.1.0 (clang-802.0.42)
configuration: --prefix=/usr/local/Cellar/ffmpeg/HEAD-017a68e --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --disable-indev=qtkit --enable-libfdk-aac --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --disable-lzma --enable-nonfree --enable-vda
libavutil 55. 63.100 / 55. 63.100
libavcodec 57. 96.101 / 57. 96.101
libavformat 57. 72.101 / 57. 72.101
libavdevice 57. 7.100 / 57. 7.100
libavfilter 6. 90.100 / 6. 90.100
libavresample 3. 6. 0 / 3. 6. 0
libswscale 4. 7.101 / 4. 7.101
libswresample 2. 8.100 / 2. 8.100
libpostproc 54. 6.100 / 54. 6.100
Input #0, matroska,webm, from 'titleall_no_meta.mkv':
Metadata:
ENCODER : Lavf57.72.101
Duration: 00:03:40.02, start: 0.000000, bitrate: 3021 kb/s
Stream #0:0: Video: mpeg2video (Main), yuv420p(tv, bt470bg, top first), 720x576 [SAR 16:15 DAR 4:3], 4500 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc
Metadata:
DURATION : 00:03:40.000000000
Stream #0:1: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s (default)
Metadata:
DURATION : 00:03:40.016000000
ffprobe output of the ffmpeg concatenated file (with metadata removed):
$ ffprobe titleall_no_meta.mkv
ffprobe version N-46247-g017a68e8f Copyright (c) 2007-2017 the FFmpeg developers
built with Apple LLVM version 8.1.0 (clang-802.0.42)
configuration: --prefix=/usr/local/Cellar/ffmpeg/HEAD-017a68e --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --disable-indev=qtkit --enable-libfdk-aac --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --disable-lzma --enable-nonfree --enable-vda
libavutil 55. 63.100 / 55. 63.100
libavcodec 57. 96.101 / 57. 96.101
libavformat 57. 72.101 / 57. 72.101
libavdevice 57. 7.100 / 57. 7.100
libavfilter 6. 90.100 / 6. 90.100
libavresample 3. 6. 0 / 3. 6. 0
libswscale 4. 7.101 / 4. 7.101
libswresample 2. 8.100 / 2. 8.100
libpostproc 54. 6.100 / 54. 6.100
Input #0, matroska,webm, from 'titleall_no_meta.mkv':
Metadata:
ENCODER : Lavf57.72.101
Duration: 00:03:40.02, start: 0.000000, bitrate: 4696 kb/s
Stream #0:0: Video: mpeg2video (Main), yuv420p(tv, bt470bg, top first), 720x576 [SAR 16:15 DAR 4:3], 4500 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
Metadata:
DURATION : 00:03:40.000000000
Stream #0:1: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s (default)
Metadata:
DURATION : 00:03:40.016000000
There is a difference in the two ffprobe outputs:
For the MKVToolNIX concatenated file ffprobe says "bitrate: 3021 kb/s" and for the ffmpeg concatenated file "bitrate: 4696 kb/s".
I haven't noticed this earlier, and to be honest I have no idea what it means (what is this value compared to the "4500 kb/s" in the stream?)
Regarding the duration of the concatenation (1s), I doubt that the video streams were reencoded on my Macbook Pro Late 2011...