This has always been the case on all my cars.
In our Ateca it’s separate, i can heat the mirrors individually. The problem is that i usually forget to turn it off and only notice it once i sit at a traffic light wondering why the mirrors are “smoking”.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.