The 5x V006 PCIe riser boards didn't even power up the GPU's (tried direct 4pin molex and sata to 4pin adapters and even new USB cables) so i suspect that entire batch was faulty but the V007S ones all worked without issue.
I actually have a B350-BTC where I'm fairly certain I'm having the same riser issues as you. Curious where you got your batch of V006 risers from, and where you ordered the working replacements.
I have a new set on the way of 006C's as well to try, but wouldn't mind stocking some known working spares to have on the way just in case.