I think there is not enough power and your psu just shutdowns
because sometimes amd settings for downvolting are not applying. and it necessary to move the clock slider in afterburner and then hit apply and here we go voltage is now correct, so maybe you could check there?
^^^This.
Down-clock and see if it helps, I had the same issue, it worked for me.
Otherwise add another PSU or replace with stronger ones.
I just tested this, it's very little difference. Even increasing voltage from 815 to 850 still caused the same effect, and still a good 400W below the 1000W max.
I'm trying a new power setup. Different outlets for each PSU. If I'm still having problems, I'm going to switch the two psus around, because they're the same.