The issue of the door mirror glass falling out does seem to be quite common - it gets quite a few mentions on various VW forums. A few people who’ve experienced this leave their rotary mirror adjustment knob on the ‘heating’ setting all the time. I don’t know if the heating automatically switches off after x minutes, but if it doesn’t, then for anyone who does leave the adjustment knob in the ‘heating’ position, continuous heating of the mirror glass might degrade the adhesive on the fixing pads over time.
IMHO the VW door mirror heating function is a poor design. Non-VW’s that I’ve owned in the past have had a warning light that illuminates when the heating function is being used, acting as a reminder to switch off the heating when the mirrors have cleared.