I think an 80 plus platinum PSU efficiency is around 92%
6x120 watt is on verge of your PSU limit and it's natural to give up and lose GPU.
Either lower your power limit or use higher PSU
I think there is some confusion here. Only 6 risers are powered by the EVGA. ALL cards (6+2) are powered by the 1400W PSUs. 7 (6+2) on one 1400W, 6 (6+2) powered by the other 1400W. The X-11 breakout boards are connected via a Molex to the mobo so all PSUs power on/off at the same time.
I think it is best to power a GPU and its riser from the same PSU.