After lots of digging and two reoccurences, I believe I found the cause. It looks like my phone ended up in a boot looping state, where it was connected to power but the battery could not get enough juice to boot into the system before losing power again.
That's what probably killed it three seperate times.
My solution:
- ALWAYS pause at boot if your phone dies completely, to charge up the battery before booting to avoid a bootloop
- NEVER leave your phone unattended when charging from 0%
At least I have good backup hygiene now :)