The same issue on Gygabyte Z170 Gaming K3 and 6 cards Aorus RX570 4gb.
Windows 10 is working good with only 5 and with 6th is reseting.

All checked
- other risers - not work
- other power suply - not work
- other (same) mMotherboard - not work
- other 6 cards - not work.

 Did any of You overcome the problem?