If the file contains a TPE2 tag it will be visible in that field, but you will edit it via the Album artist field.
The field itself will be read-only.
If you have "Save album artist with full external compatibility" set then Band/Orchestra will be disabled and automatically assigned the value of album artist.
You can find more details here: http://support.imploded.com/solution/articles/9000082389-options
>>And another little bug maybe:
Thanks, we managed to reproduce this one and will analyse it further.