First off I'm not decrying anyones opinion on this as only time will tell if they've been built to withstand this, but every single pair of over ear and on ear headphones I've ever owned would have had the same thing happen, if the ear pads are damp when you take any closed back headphone off there's gonna be moisture in the cans, it's just usually you can't see it.
It's a closed system and even when your sitting still your body regulates it's temperature by evaporation (Sweat) you could be bone dry and you're still loosing moisture, given the APM's close in and surround your ears (a generally damp area of the body due to the ear canal) there's nowhere for that moisture to go but into the headphones as they're closed back that's just facts.
So far I've seen the moisture in my APM's when wearing them for extended periods and it seems to form on the plastic inner cup just behind the magnetic outer cup, there's a mesh membrane behind the plastic cup that I'd assume stops the moisture getting all the way in, that's not to say no moisture will get in but it would probably evaporate between uses just like most closed back headphones.
Another point to note is most people won't have ever seen behind the ear pads before, APM's are one of the few that let you detach the pads.
Hopefully Apple have thought about this over the long time they took to develop & test these (4 years+ by some reports) it'd seem daft if they didn't, i'd expect if there is a design issue we'd have seen a lot of failures by now and that would be getting reported not only in forums and tech sites but the mainstream media, but so far nothing.
I'm not worried about it, but I'll reserve judgement long term on if this is an issue at which point Apple would probably be forced to issue a recall/replacement just like with Rattle-gate on the AirPod Pro's. ?