Funnily enough, I think I've realised my issue is unrelated; too.
I seem to be hitting: https://pve.proxmox.com/wiki/Upgrade_from_6.x_to_7.0#Older_Hardware_and_New_5.15_Kernel
I suspect there's something being fired out of cron in the guest at the exact same time causing the bug. Correlation !=...