Comment # 6 on bug 1090458 from Takashi Iwai

Well, the non-existing firmware isn't a problem.  The driver supports a new API
version 6 while it falls back to API version 5.  Currently the API v6 firmware
isn't publicly available, as it seems, so the behavior you've seen is correct.

Now I took a look at the recent ath10k1 development, I found a fix in 4.17-rc1
that is likely relevant with your problem.

55cc11da69895a680940c1733caabc37be685f5e
    Revert "ath10k: send (re)assoc peer command when NSS changed"

I'm going to build a kernel with this fix for testing.