That’s quite strange; does it happen in all slots?
Assuming your rollers are clean, your filament and dessica t moisture content are adequate, no mods, and reasoning on physics:
- When you print, the air temperature within the AMS is warm;
- During the idle period, the temperature decreases, which can induce a slight negative relative pressure within the AMS, which could hypothetically induce compression over the cover.
But this would require large temperature gradients. It is easy to test, as when it happens in the AMS 2 Pro, you can try to heat the filament and then retry.
These are guesses, and I have never experienced a similar situation with a standard spool size…