can I ask how you prepared your S21u phones each time ?
Did you synchronise all of your previous information from old phone to your s21 using Samsung switch tools ?
and then after sending the first one back you synchronised the same data to the second one?
I am no expert and undoubtedly there are some handsets out there overheating more than others.
my experiences with overheating effects were fixed by carrying out a factory reset and manual installation of apps.
my original synchronisation of my 400 + apps between android 10 and android 11 caused me with no end of problems, and unknown battery drain. My battery life was rubbish and the phone was heating at random times...
After factory reset and no synchronised apps and data... the experience was completely different.
I am not suggesting for a minute that this is your case... but if you did perform smartswitch then you may find all your problems are resolved by trying this.
Smiley.
- Smiley
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SAMSUNG S23 ULTRA 512GB SM-S918B/DS
Samsung Galaxy Watch 5 Pro SM-R920 in Grey Titanium
Samsung Galaxy Buds2 Pro SM-R510 in Graphite