It sounded to me like they are encoding lossless WAV to lossy MP4, and replacing MP3s that were also from the same WAV, and in any case also keeping the WAV. T

It sounded to me like they are encoding lossless WAV to lossy MP4, and replacing MP3s that were also from the same WAV, and in any case also keeping the WAV. That's why it was notable that in a couple of cases where the WAV was corrupt, they kept the original MP3 as being now the 'best available' copy. In no case did they transcode MP3->MP4.