I have been experiencing this issue with several linux distros(Ubuntu,Pop,Arch) that , my laptop shuts down randomly.
Its like One second I am working on it and the next second it shuts off, like no shutting down or whatever the shutdown sequence is it just skipped and it is as Off it can be.like completely blank screen.(If this fact matters to anyone my laptop battery has died and it doesn’t charge anymore it needs to be connected to Electricity to work.)
I did a bit a of googling and found out that this may be a overheating issue(that maybe my fans are not working) but my windows works perfectly fine(I am dual booting).(Also there is a setting enabled i my bios which sets fans to always ON.)
It also should not be a bootloader issue as from what I understand popOS uses a different bootloader and not grub and it still had the same issue.
If this is any helpful there were no issues with Ubuntu 18.04(I don’t remember the exact version) although i was due some updates at that time. The issues started when I installed Ubuntu 20.04 in its place and the issues have continued.
After this I installed arch,pop and then manjaro but the issues have continued.