I had the same problem with mine running the latest version of the default installation. If you left it without the power adapter plugged in for 5-10 days, let it charge with the appropriate rated charger for 72 hours or more.
I unplugged mine while I was gone on holiday and came back to what appeared to be a dead tablet, no response to anything (odd since I hadn’t used it, it was shut down and unplugged).
I tried the UART adapter- nothing, charged it for 24 hours- same, then took it apart, re-seated all the cables & still nothing. I put it all back together, left it on the charger for a few days and then it was back to working as normal.
I never did figure out what drained the battery.
PinePhone, Pinebook Pro & PineTab2 owner.