Problems with sample encoded with opus
source1 (mp3@256): Danny Keith - Keep On Music.mp3 fragment* [0:35-0:53]
source2 (opus@160): https://www.youtube.com/watch?v=qlugmsrLvpI [0:44-1:02]
mp3@256 -> opus@48, uses higher bitrate (~60 on this part) but still sounds bad
mp3@256 -> opus@64, uses 80kbps on problematic sample, sounds better but still not good
mp3@256 -> opus@80, uses 105kbps on problematic sample, sounds reasonable
opus160 -> opus@80, still seems a bit bad, maybe acceptable
In the past I found out same sample/instrument in different songs with same problem.
Opus uses high bitrate for this sample but still sounds bad. Either should improve encoding of this sample/instrument, or raise bitrate even more.
* - cut from full song using mp3directcut (lossless)
When reporting problematic samples, It only makes sense if you compare to the lossless original. Converting lossy to lossy, no matter bitrate, is likely to produce artifacts. In your case, mp3 might have thrown out information opus could have used to create a better result.
I do not have access to lossless original for this song. But doesn't seem to influence much, as same song encoded with FDK AAC @32kbps sounds superior to opus (on the problematic fragment) which uses much bigger bitrate.