Not sure this will be relevant but your system of 12 cards using one 850 psu for the risers is how I killed two psu. I only had 7 cards running and it was too much for the Sata channels to handle. One psu will be good for 6 sata devices so I think you are browning out the main psu. Once I dropped back on card count, no more problems. In the end, I gave up on the Server psu's and went with dual 1000w psu's.
thay
The 850W PSU only runs the motherboard and one card and it's riser. I have three PSU's per rig. Two 1600W (6 cards and their risers each) and the 850W as described. I am no were near the amount of power the power supplies are capable of, so should be good there.
This is one of the rigs -
https://imgur.com/a/WRfTksJThat is a really nice (and big) rig. Did you find anything in syslog (as per my previous post)?