iPhone Strange Wifi Logging Out? What's Going On Here?

I'm using an iPhone 6+ on my home network (Airport Extreme v4). Lately, the phone is logging out of the home password protected wifi network by itself. The strange thing is that I recently reset the Airport (complete rebuild) and now just THIS phone is logging out. Now here's the really weird thing. The iPhone can be logged back in with either the old wifi password or new wifi password, but only every other time.

Other iPhones on the same wifi network are NOT doing this, fyi.

I'm tired of entering alternating passwords. But mostly, I'm tired of wasting data plan when it should be on the network.

How's that for weird?

Any advise?