I suspect the issue may be the heating and freezing of the adhesive over time causing expansion and contraction.
Yep this is exactly what I'd say too. Plus the fact that it's clearly not fixed on as strong as it should be from the factory if there are this many issues. And to settle the argument, mine just randomly fell off on a sunny day after the car was left sitting for a few weeks during the first lockdown, heating function wasn't on.
Not sure why people are even considering using glue, thats just mad!. What happens if the glass cracked and you needed to replace it? You'd rip the heating pad apart. If anything use an extremely thin layer of clear silicone as that can be removed.
Also if you use an adhesive backed mirror the area you're sticking it to should be cleaned and dried before applying