I've had similar issues with server psu's and modular psu's working together to power a rig, or powering multiple setups..
Somethings not right with server PSU's when it comes to powering risers. Not sure what exactly; I have setups with and without risers and powering them w/ a server psu sucks ha.
So, I started powering the risers, mobo, and CPU with a 650w fully modular psu..leaving the server psu to do all the heavy lifting with GPU's. Not sure if it's just me.. but switching my power source from server to modular for merely power the risers, solved my glitchy power issue.
I really feel we've had the same issue. Hope this works out for you.
that was my setup as well, but i did not use the modular PSU to power the risers , I think it was the motherboard after swapping it out the machine seems more stable no shutdowns, and when i power it off the fans spin on some cards like @PcChips said
but they are still detected as normal.