I went to my phone provider, brought the phone and I even one of my chargers to do a "live" test. I didn't bring the one with 100 watts 'cause I fear they would blame it on it being "too powerful"(and be responsible for the overheating) and the guy commented something about the 35 watts charger that is "meant for Ipads" (no idea if they actually know what they're talking about but I brushed that "argument" off right away).
Anyway, told them I got the same issue with both chargers, and that I got the overheating message during data transfer as well Day One (some of their colleague even said this was "normal" - yet I never experienced this with past Iphone Prox Max... No idea where they got this from).
The staff member then did some reset (the one that doesn't erase your data) and while it was charging, the phone still got pretty hot (just like at home), despite them trying to make it look like it was acceptable. The colleague brought up that many people had this heating issue since IOS 26... There are not Apple employees so they couldn't tell if a future update could fix it.
Speaking of which, the 26.0.1 update I did last night also made my phone overheat in the process... and didn't fix the issue afterwards. Long story short : they told me to wait 48 hours and if it doesn't get any better, come back and send it to "repair" which is what I didn't like about their solution. They won't replace it with a new phone and would only land me some Iphone SE in the meantime (don't even know what it is but I guess some burner phone version from Apple) while mine is getting "repaired" (whatever that means)...
Not sure what to do....Clearly disappointed and I regretted having sold my 16 Pro Max that didn't have any overheat issues. I guess this is my lesson.