I have almost same problem. My rig was worked fine about 1week. Later something went wrong... I started search why my pc don't see only 6gpu of 7. Started to check cables, and found that cable which goes from psu to riser burned one contactor of six. Ok I thought maybe it's because I using cable for more then one year. I plug it to other psu place and turned on with -starp 1. Again my rig started work well but only about one hour and again it started to restart and again one gpu lost. Again checked cable and again same problem. Then I tried again to change in psu cable place and turned on rig without -strap function and currently everything is ok. I am not sure but I think something not so good with that -strap function. My rig working on two psu  which is corsair RM850X. Have someone same issue?

Thank a lot!