pomon Sorry, I posted then left for Holidays, so haven't had a chance to come back to this yet. But now I have some time, and would really love to debug this if I can.
It is happening on the current kernel. Grub tells me it is 5.6.19-158.current. I haven't tried it on the lts. I'll do that now.
To check my sanity, I tested it again before I left - wiped the Hard Disk, then reinstalled Solus from scratch, rebooted, closed the lid on my laptop, unplugged it, and left it while we were away, I come back (the battery is well and truly dead), so I plug it in and try to boot it up, and still have the same issue.
I've tried multiple other distros (openSUSE, Debian and Arch), and none of them experience this behaviour, so Solus must be doing something different to the rest.