Thanks for trying, I guess that means it's not a bug, and should generally be working just fine, so this issue is limited to my setup only.

I already suspected the power supply (since it's a PicoPSU), but the green onboard standby power LED above the power connector is on at all times, so even when the computer is turned off, as well as when going to suspend, and during a failing resume.

I'll nevertheless will try a normal fullsize power supply as well, but if that also does not change anything, the last option is to assume that this motherboard is faulty, and has to be replaced.

I'll close the opened bug for this issue now, and will report back with further results.