Or here is the action that can add the desired spelling:
Action type: Guess values
Source format: %bpm%
Guessing pattern: %fBPM%
If there is already a fBPM field in upper case it might be needed to remove it first.
Or these two can be used to fix it afterwards:
Action #1:
Action type: Guess values
Source format: *%fbpm%
Guessing pattern: %fbpm%
Action #2:
Action type: Guess values
Source format: %fbpm%
Guessing pattern: *%fBPM%
Or last possibility is to add the field to the tag panel (Options > Tag panel) and edit the usrfields.ini as explained here:
/t/3738/1
Plenty of ways to fix it...