That rule is nothing to do with JAR-FCL - it is down to a CAA cock-up. The drafter of the ANO didn't understand the JAA requirement properly and made the original ANO error - now the CAA's compensating error is to expect people to wait until three months before the expiry date (even though they may have met the requirements months earlier) for a signature.....
Thank you for the reminder - I will remind the CAA of this cock-up on Monday and ask that they correct it in the next ANO amendment!