Please test the Xubuntu laptop system with exhausted notebook battery plugged. It should outlast it. My notebook battery was unattached for many months. (I hadn't know that this can cause the battery to "die".) I attached the battery. The system booted and behaved for a while normally, except that the orange battery led (which, I guess means charging and normally lights steadily) flashed (twinkled). After a while I opened the power manager, and saw that the charge level is 0% and some "estimating" or "calculating" label, regarding (I guess) the capacity and/or current charge level. Then (not many later) the laptop became unresponsive. No mouse, no keyboard. Even the Ctrl-Alt-F3 key combinations did not work. I pressed for long the power button.
From what you reported I see no evidence to support this as an xfpm bug, it can be pretty much any other component or even a hardware failure. You may try to disable xfpm (at Session and Startup) and see if the system is stable. You can also try to read system logs (journalctl, dmesg) and look for something suspicious.
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