Ok, so this is bugging me ;-)
If you could do me a favour:
Download MusicBee portable:
https://mega.nz/file/MxETQAxS#3gaBLs4InYbIYDmxA_wDYJzFeI-FIcC6ffQo7lljUmg
and install it somewhere outside Windows' system folders. (preferably on some D: or E: drive)
(it won't interfere with your current MusicBee install in any way)
- create the custom tag:
- save and restart MusicBee
- create the virtual tag:
$If($Contains(<Picard title full>,₁)="T",$Split($Split(<Picard title full>,₁,2),¹,1),)
Does creating the virtual tag then still trigger that exclamation/warning?