OpenSUSE's kernel is built with CONFIG_RESET_ATTACK_MITIGATION=y, see bug
1108302.

However there is no accompanying userspace component to tell EFI that the
system was shut down cleanly as recommended in the kernel help for that option.
The mitigation causes unusual reboots at least on my hardware (and I've seen
other bug reports of similar behavior, e.g.
https://bugzilla.redhat.com/show_bug.cgi?id=1532058).

This could be resolved either by adding a systemd unit as described in the RH
bug I linked or disabling this option.