Billed Out of Network for In Network Provider?

I went to the dentist back in February. Got an email just an hour ago about a bill? This was my first time seeing anything. I log into my dental insurance and see that I owe $400?? All I got was a routine exam. I call my insurance and they said I went to an out of network provider which is why I was billed. I checked on my insurance site AND called the dental office to confirm they were in network before my appointment, the site and dental office said that they were in network. Told my insurance customer service rep this and she looks up my doctor and agrees that he is in network. She is reprocessing my claim as in network and now I have to wait ten days to see what the outcome is.

I’ve never had to deal with this before. I actually have an appointment coming up with this same office (6 month routine check up) and I’m not sure I should go back to this place until this claim is resolved. Can anyone tell me what to expect?? Should I hold off on my appointment until I see the updated claim?