I carefully charged a Motorola G Power 2020 to a 60% SoC (which should be very safe) and left it in the car, unplugged, as a spare. It puffed up in about 6 months.

An ignored Galaxy S7 puffed up while sitting on a shelf, unplugged, in a climate-controlled room.

Some other devices have been fine for far longer periods so it's not like I'm thinking it's endemic or anything (my Sony PSP is still working well-enough with its still-flat lipo cell from 2008), but it's certainly a concern.