I recently installed a new 8500NEX and had no problems setting up the WiFi connection for both my and my wife’s iPhones. Last week, my phone died and was replaced with a new one. After performing the restore, the 8500 network credentials were still in the phone, but it wouldn’t connect to the HU. It has a check mark next to the 8500 in the network list, so it seems as if it’s connected, but it’s not. I figured it was because of the different MAC address of the new phone so I had it “forget” the network, reset the 8500 password and RE-joined the 8500. Still won’t connect (as far as CarPlay is concerned). I opened the device list in the 8500 and told it to search for new devices and it doesn’t find the new phone no matter what I do. 

Any suggestions short of a of a factory reset?

Not sure if you’ve found your answer yet, but this happened to me when I went from an iPhone 6 to a Xs on an 8000.

Have you tried deleting the phone profiles off the 8500 and then re-connect? This is what worked for me.