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.
Have you recently done any hardware modifications? I had very similar issues which persisted until I realized I had not fully seated the 24pin connector after changing out my CPU cooler. After seating it properly, random boots and reboots went away.
Haven't opened the case in months