I have found the problem! Silly me. I have the following action which is above my new action which strips the comment field if it doesn't start with a digit as I always format my COMMENT tag like '4A - 5' 4A being the Camelot Notation and the 5 the rating of the track.
Action type: Replace with regular expression
Field: COMMENT
Regular expression: ^[^\d]+.+
Replace matches with:
[ ] case-sensitive comparison
So for anyone out there who wants to change the INITIALKEY tag from F♯ Major to 2B and append it to the start of the COMMENT tag then here is the working action.
Name of action group: For&mat#INITIALKEY Key to Camelot Notation
Action #1:
Action type: Replace
Field: INITIALKEY
Original: B Major
Replace with: 1B
[ ] only as whole word
[ ] case-sensitive comparison
Action #2:
Action type: Replace
Field: INITIALKEY
Original: F♯ Major
Replace with: 2B
[ ] only as whole word
[ ] case-sensitive comparison
Action #3:
Action type: Replace
Field: INITIALKEY
Original: D♭ Major
Replace with: 3B
[ ] only as whole word
[ ] case-sensitive comparison
Action #4:
Action type: Replace
Field: INITIALKEY
Original: A♭ Major
Replace with: 4B
[ ] only as whole word
[ ] case-sensitive comparison
Action #5:
Action type: Replace
Field: INITIALKEY
Original: E♭ Major
Replace with: 5B
[ ] only as whole word
[ ] case-sensitive comparison
Action #6:
Action type: Replace
Field: INITIALKEY
Original: B♭ Major
Replace with: 6B
[ ] only as whole word
[ ] case-sensitive comparison
Action #7:
Action type: Replace
Field: INITIALKEY
Original: F Major
Replace with: 7B
[ ] only as whole word
[ ] case-sensitive comparison
Action #8:
Action type: Replace
Field: INITIALKEY
Original: C Major
Replace with: 8B
[ ] only as whole word
[ ] case-sensitive comparison
Action #9:
Action type: Replace
Field: INITIALKEY
Original: G Major
Replace with: 9B
[ ] only as whole word
[ ] case-sensitive comparison
Action #10:
Action type: Replace
Field: INITIALKEY
Original: D Major
Replace with: 10B
[ ] only as whole word
[ ] case-sensitive comparison
Action #11:
Action type: Replace
Field: INITIALKEY
Original: A Major
Replace with: 11B
[ ] only as whole word
[ ] case-sensitive comparison
Action #12:
Action type: Replace
Field: INITIALKEY
Original: E Major
Replace with: 12B
[ ] only as whole word
[ ] case-sensitive comparison
Action #13:
Action type: Replace
Field: INITIALKEY
Original: A♭ Minor
Replace with: 1A
[ ] only as whole word
[ ] case-sensitive comparison
Action #14:
Action type: Replace
Field: INITIALKEY
Original: E♭ Minor
Replace with: 2A
[ ] only as whole word
[ ] case-sensitive comparison
Action #15:
Action type: Replace
Field: INITIALKEY
Original: B♭ Minor
Replace with: 3A
[ ] only as whole word
[ ] case-sensitive comparison
Action #16:
Action type: Replace
Field: INITIALKEY
Original: F Minor
Replace with: 4A
[ ] only as whole word
[ ] case-sensitive comparison
Action #17:
Action type: Replace
Field: INITIALKEY
Original: C Minor
Replace with: 5A
[ ] only as whole word
[ ] case-sensitive comparison
Action #18:
Action type: Replace
Field: INITIALKEY
Original: G Minor
Replace with: 6A
[ ] only as whole word
[ ] case-sensitive comparison
Action #19:
Action type: Replace
Field: INITIALKEY
Original: D Minor
Replace with: 7A
[ ] only as whole word
[ ] case-sensitive comparison
Action #20:
Action type: Replace
Field: INITIALKEY
Original: A Minor
Replace with: 8A
[ ] only as whole word
[ ] case-sensitive comparison
Action #21:
Action type: Replace
Field: INITIALKEY
Original: E Minor
Replace with: 9A
[ ] only as whole word
[ ] case-sensitive comparison
Action #22:
Action type: Replace
Field: INITIALKEY
Original: B Minor
Replace with: 10A
[ ] only as whole word
[ ] case-sensitive comparison
Action #23:
Action type: Replace
Field: INITIALKEY
Original: F♭ Minor
Replace with: 11A
[ ] only as whole word
[ ] case-sensitive comparison
Action #24:
Action type: Replace
Field: INITIALKEY
Original: D♭ Minor
Replace with: 12A
[ ] only as whole word
[ ] case-sensitive comparison
Action #25:
Action type: Format value
Field: COMMENT
Formatstring: %initialkey% - %comment%
Action #26:
Action type: Replace with regular expression
Field: COMMENT
Regular expression: ^\s+-\s+|^\s+-\s+$|\s+-\s+$|\s+-\s+\d{1,2}[A-B]\s+-\s+$|\s+-\s+\d{1,2}[A-B]$|^(\d{1,2}[A-B]\s+-\s+)\d{1,2}[A-B]\s+-\s+(\w+)
Replace matches with: $1$2
[ ] case-sensitive comparison