Drivers, Bad setup, something else … Too many variants for this mobo
May you prompt how to check riser quickly? I suspected one of them. Perhaps, is a tool?
For riser - only eyes or hardware tools )
Usually when one or more cards cause errors, we have corresponding error records in dmesg. So, check dmesg output
Dmesg has not an error or warning
12 or more GPU rig is a big headache.
Russian saying says “You love to ride - love and sledge carry”.
Yes, diag problems in big rig problematic.
Try to diag half of them then another half