Let assume the upgrade worked before..then the solely posibility that the AP revert his code back is
if he resets 3 times . In this case the AP will take his 2nd image / backup image.
If you upgrade the AP twice then yo will have the same code as your primary code and for your secondary / backup code.
In this situation it shouldn't back after a reset.
FYI.
But till get this approved it's the best you try it wit controlled upgarde.
If your AP are behing Cloud/Internet/VPN reduce the MTU size to 1400.
Maybe tftp(upgrade) is maybe forbidden or the MTU size was to big?!
Good luck
Umut