Laptop reboots instead of resuming from systemd suspend when on battery power (suspending on AC power works)
Looking into the BIOS options, I found "Wake on lid open". Even if it says "Enabled" it does not always work.
I found that when I upgrade the kernel, I have to go into the BIOS, select "Disabled", reboot, go into the BIOS again and select "Enabled".
Amazingly, with this process the laptop wakes from suspend on battery as well.
It's actually a Kernel Bug, see Kernel Bugtracker here: https://bugzilla.kernel.org/show_bug.cgi?id=108801