This first happened earlier this week (after updating the system last weekend), and for the second time now. Looking it up the general consensus seems to be that any random reboots are a hardware issue, but I've been running this rig (7950x) without issues for years and there are no errors in the journal so I'm skeptical.
I'm not sure if calling what's happening a reboot is correct neither, one second it shows the os, next second the bootloader without the usual indicators that it turned off and on again.
Last time the journal was very clean with a clear separation between the new boot and the old boot but this time it happened when opening a game from steam so the messages from the two boots are printed interleaved (although the timestamps aren't). There's exactly 0.99 seconds between the last message from old boot and the first message from new boot. What's stranger is that my systemd-boot waits for 3 seconds before booting..
I'm asking here to see if anyone else had the same issue and knows what's causing it. I updated the system just now to see if it will help.
Edit: Fuck me, after the update random ui elements (like the Proton version selector on Steam) started spasming. It feels like 2023 wayland+nvidia again
Edit 2: Lsi steam works
Edit 3: It didn't happen since posting this so I assume updating fixed it.
You sure it was showing the bootloader and not just kicking you back to your login screen?
I'm pretty sure, at the very least it showed the boot crawl