The cheap ZTE devices in particular seem especially bad at keeping the thermals within a safe range. Even with a 100% full battery they'll let the electrolyte boil if left plugged in for their charging logic to do as it wishes in a hot environment.
What's so frustrating is practically every consumer electronics device seems to now have some form of LiPo pouch cells in them. I wish it were normal for manufacturers to offer chunkier variants using NiMH AA/AAA cells to consumers. Instead I've been resorting to "industrial" stuff having no battery at all like the Gl-Inet X300B [0] I eventually ended up with.
For a while I used a hacked together pack of four NiMH AAs with a battery board stolen from the LiPo on a ZTE/AT&T hotspot that kept swelling its pack. The battery board kept the device happy enough to believe it had the proper LiPo connected. The now external AAs stayed charged more-or-less, and didn't care how hot the ambient temps got through summer. But it was a sprawling mess of wires, soldered AAs arranged like an 80s-era RC-12L saddle-pack, kept ~together with Duck tape.