Yeah, iTunes is the same way... I really wish MP3Tag would just leave the tags as you typed it in...
In the meantime, you have to run an action to make the case as you like it as follows (in my example, I "correct" iTunEXTC's case:
"Regular Expression" --->> for "Field" type _FIELDNAME
--->> for "Regular Expression" type the tag as it is today: ITUNEXTC
--->> for "Replace Matches With" type tag as you want: iTunEXTC
Leave the "Case Sensative Comparison" checkbox UNchecked.
You can find an example in my IMDB script / action sets found here: [WS] imdb