To reproduce, open Mp3Tag on a folder with usual single-track files with track numbers filled in.
Create a column:
- Name: №
- Value: [CD%discnumber% - ]%track%
- Field: [CD%discnumber% - ]%track%
Values from this column appear to be editable when clicked (at least when only track number is present, but not discnumber). However pressing Enter doesn't save the (numeric) value as track number. Of course if Field is just "%track%", behavior is as expected (editable and saved).
I'd suggest to prohibit such optional fields since they mislead users into thinking one can edit two fields in one control.