While I haven't touched the rig myself, it's been reported to me that HBCC segmentation can cause crashing on 1x to 16x powered risers.

Myself, both of my vega cards are running 8x in motherboard slots.  Without HBCC segmentation I get 36Mh/s, with I get 42Mh/s.  This is at HBM2 clock of 1100MHz.