Perhaps probationary would be a better description of APO status.
My mate "Stan" had a similar problem to Todger in the tread
RAF Pilot Tour lengths, etc in that his fiancée decided she didn't want to be an aircrew's wife. Now Stan had joined on the 8 / 12 scheme and was no longer acting. So Stan quits flying training....but he is signed up for 12 years.....off to the Isle of Man with you young boy shuffling papers. Very inconvenient when fiancée lives in South London, so he wants to resign. Right young man you may resign but you have not completed your National Service commitment so you will make up that time as a/c2 where ever we send you. Immediate change of heart....Please can I go back to Nav school? That's what happened but he won in the end, he got out after a little over 2 years but I think it cost him a bit.
So perhaps the substantive rank is a/c 2 even if you are no longer acting.