Not correct. Fairphone anti rollback is specifically triggered by locking only.Without locking rollback errors if existing are ignored.
The two dates for comparison are
- Android SPL of the build where the phone was last locked (for a new install this will be the Android SPL at time of unlocking).
- Android SPL of the build you intend to lock.
There must be “no going back” between dates.
The full rules are copied here (answered) Install /e/ on FP5 with newer Security Patches - #2 by aibd.
The fix was more complex than that. You will have to do more reading (perhaps track up to the start of that thread) if the phone is not in the correct state to lock. There are 6 or 7 late locker threads if the explanation is unclear at first pass.
Ofc from here I do not know from here the actual cause of your problem.