Please describe the process step by step so that others can reproduce it even if they can't see your screen.
Start with the description like this:
a) I load an album with 12 mp3 tracks into Mp3tag and see this:
-> Show us a screenshot
b) Then I click with my mouse on track #3 and this line "go blank"
-> Show us a screenshot
This will help us to try and imagine what could happen in your case.