This is nothing new, I was pinged for this back in 2004 and had to pay back £3,000.
It was all a huge balls up as we were told we could use an AIP for the Quals you got from JMLC (the instructors on the course told us all to apply for it if we hadn't used the AIP already). What they didn't say was only some of the Trade Sponsors had signed it off, all the paperwork went in and was authorised and i got my AIP.
Three years later as in my first pay statement in Akrotiri was a reclaim of £3,000. I found out later aircraft trades apparently didn't qualify for the AIP as our trade sponsor hadn't authorised it (MT, Suppliers, Admin all did). Nobody could answer the question of why some trades got it and some didn't and why there was no parity across trades, etc.
I tried fighting it and went legal and the result was the same i owed them £3,000.
If the unlucky 2,000 who have been pinged on this win their fight I'll start again and ask for my money back as well.
The whole Pay 2000 and AIP thing has been a mess from day one.