The implementation of the GNOME interface variant of LibreOffice has that issue under Qubes OS. The GENERIC (looks Motif style) or the KDE implementations do not have that issue. I simple switched to the KDE version and it works great.
I used to have daily freezes as you describe (high load situations during backup or salt-based updates) with kernel 5.4 under Qubes OS R4.0 and had to stay on kernel 4.19. With Qubes OS R4.1 I am now on kernel 5.10 and have not seen any freezes yet. My laptop is a T430 with an i7-3840QM. I think it’s very possible your situation might be similar (kernel version related).
I’ve never seen any random or abrupt shutdowns, except when I created them myself by misconfiguring qubes-shutdown-idle. Have you played with that?