Insurance says my therapist is out-of-network but she's listed in-network on their own website - claim denied anyway

I've been seeing the same therapist for about 14 months. When I started, I verified through my insurer's online directory that she was in-network for my plan with BlueCross. I confirmed it with her office too - they ran my insurance before the first appointment and confirmed in-network status. I've paid my copay of $35 every session without issues.

Last month I got an EOB saying three recent sessions were processed as out-of-network at 50% coinsurance instead of my copay. I owe $620 suddenly. I called BCBS and they said her network status changed and she is no longer in-network "as of 90 days ago." No letter. No notice. Nothing. She's still listed as in-network on their website as of today - I'm looking at it right now as I type this.

I feel like I have a legitimate case here. I relied on their directory, continued care based on that information, and they still haven't updated the directory to reflect the supposed change. Has anyone won an appeal in a situation like this? I'm specifically looking for language around "directory error" and whether that creates any kind of liability on their part.