Use a reliable, popular, widely regarded and easy-to-use free 3rd-party tag utility such as MP3Tag
, to examine your MP3 files. Using Windows components, players, Explorer, etc., can often mislead you or hide what's really going on. Same with other all-in-one products (e.g. Media Monkey) which are powerful, but "intrusive" and somewhat mystical. You get much more useful and manually controlled results using specific 3rd-party tools such as MP3Tag.
You will see whether or not there truly are valid tags inside of your MP3 files. If not, you can easily manually edit/repair them with MP3Tag... although this doesn't explain where they might have disappeared to, or were actually stored when you built the MP3 files in the first place.
I'm not familiar with iTunes (don't use it, never have) so I don't know what it might have done. I build my own MP3 and FLAC files from my own CDs using Audiograbber and LAME, or Audiograbber and FLAC-Frontend. Tags are 100% properly imbedded in all files by either process.