Another reason could be that the CPU being pushed to it's limit could be the reason because I tend to notice the heat whenever I play a game on my ZMax Pro and it does literally turn into a hand warmer even if the charger is unplugged. And in this case, the heat can also be felt on the top of the screen. PCs and laptops have big enough heatsinks and often with a fan, most phones don't have much in the way of cooling and that's why they get hot in the hands too
I have been noticing this issue with my Z981 for the last few months, OTA update isn’t the issue as I never had it activated after I got it from a family member back in December 2018 and I’m sure that installing TWRP and Magisk isn’t easy or possible on this phone And it does boot loop like once or twice a month as well
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