So, I discovered some new things. Every song for which the tags are not loading have a .lrc (synced lyrics by minilyrics) file associated with them. I also noticed that a minority of tracks with a .lrc file still worked anyway. Finally I noticed that my most recent tracks in my library all work even if they had a .lrc file.
I tried a few things. First I tried deleting the .lrc file, which didn't work. Then I tried overwriting the .lrc file from within minilyrics, which actually made the info appear in Subsonic!!! Upon closer inspection I finally realized that it actually had nothing to do with the .lrc file exactly, but rather "ID3v2 synchronized lyrics."
I remembered that I had updated minilyrics recently for the first time in a while and thought this might be why the behavior was no longer happening. So I went to the minilyrics changelog
http://www.crintsoft.com/mlwhatsnew.htm and looked to see what might have changed in version 6 or 7 that might have affected this behaviour. Turns out in a (fairly) recent version there was this entry:
Fixed: Bug of saving ID3v2 synchronized lyrics (Compatibility with Windows Media Player)
So, I am pretty sure now that this is the cause.
I don't really want to go through my whole collection and delete all of the ID3v2 synchronized lyrics. That is unless someone knows of a program that could do this all at once. MediaMonkey is not capable of it. I haven't tried Windows Media Player, but this might be a good option since it seems to be the only program that uses ID3v2 synchronized lyrics. Does anyone know if you can delete all ID3v2 synchronized lyrics at once from within Windows Media Player? Does anyone use Windows Media Player on this forum? Does anyone use Windows Media Player period?
Regardless, Subsonic not recognizing tags because of the ID3v2 synchronized lyrics bug is a problem with subsonic. No other programs have the same problem, and it should be an easy fix.
TL;DR
ID3v2 synchronized lyrics bug is causing tags to not be read in Subsonic.
Need help from a Windows Media Player user.