-I'll download an entire album from an artist.
[It’s only when I change a tag field from X to Y in] a song in the .MP3 format [that] it will remain "X" when I export it.
[If] I were to open the "My Music\iTunes\iTunes Media\Music" folder and look at the song files, they're still written with the original song information before I imported them into iTunes.
A simple fix will be to convert songs into AAC/ALAC if not otherwise done. Though I have a feeling the information should still be writing to the ID3 tag in the MP3 file.