Specifically, by default, SoX automatically adds TPDF dither when the output bit-depth is less than 24 and any of the following are trueThis is true for SoX commandline tool. But you aren't using it.
If you use ffmpeg, it will NOT dither by default. You need to set dither manually.
More dither methods are available, see https://ffmpeg.org/ffmpeg-resampler.html