This may not be the same issue, It turned out that my mp3 files that were skipped were all “not normal” mp3 files in some way. The commonality of the skipped mp3 files seemed to be:
A file created with MPEG 2.5 Layer III, that has a sample rate of 11025.
a file created with MPEG 2 Layer III, that has a sample rate of 22050
In the end, because these were not actually quality music files where I’m worried about sound quality (lots of spoken word podcasts, or pretty bad recordings of very old radio shows from the 1950s/60s (e.g., Jean Shephard shows), I simply re-encoded these “odd” mp3 files using -V0 LAME mp3 encoder. This turned them into standard mp3 files (yes I know that re-encoding an mp3 file creates a version loss…but as noted, it is irrelevant for these files).