My understanding is that full heat acclimatization requires elevated core body temperature for 30-60 minutes a day for several weeks to really kick in and it’s easily lost during winter. Someone going largely from air conditioned to air conditioned building during the summer months may not even build up much of a passive acclimatization.

I don’t do as well in heat so for the summer hiking months in California so I have to do a two week sauna or hot bath protocol, otherwise I struggle really badly during the season. YMMV of course.