The EC can cause these symptoms.
To find out if the EC crashed, you can use the command “ectool panicinfo”.
If it has an entry in there, the EC crashed, causing the reboot / unexpected power off.
To reset the EC.
Remove the PSU and power off the laptop.
After the Power button LED goes off, the EC will reset 40 seconds later. So, leave it off for 1-2 mins, then power on.
After the EC reset, “ectool panicinfo” should be empty, until the next crash.
So, I would be interested to see if there is any ectool panicinfo output.
An example of the output is:
ectool panicinfo
Saved panic data:
=== PROCESS EXCEPTION: 00 ====== xPSR: 61000000 ===
r0 :00000000 r1 :200c738c r2 :00000000 r3 :00000000
r4 :100a6328 r5 :200c738c r6 :00000000 r7 :00000001
r8 :00000001 r9 :100afb49 r10:100a7180 r11:00000000
r12:100ad781 sp :200c7338 lr :1008b551 pc :1009f2da
cfsr=00000000, shcsr=00000000, hfsr=00000000, dfsr=00000000, ipsr=00000000
The panic info is stored in the RAM of the EC. So it survives a reboot of the CPU, but does not survive a EC reset.