I am having a very similar problem.  After the Win 10 update, I cannot get system to boot up with more than 4 GPUs.   All GPUs work fine if there's only 4.  I do see lagging and GPUs behaving weird with 4 if the BC driver installed (it looks like an issue with the HD audio driver - but disabling in the bios strangely doesn't stop it).

mobo is ASUS z270-p, and I updated bios to 1205 - still no dice.  ...and there's no way to rollback the bios unfortunately.

I guess I'll try re-installing an earlier version of windows from USB...