I don't have experience, but I'll tell ya, this is exactly why I don't even bother with pet health insurance. Inurance companies suck

How the heck would you possibly have previous records for an adopted dog? Sounds like they're just trying to weasle out of paying for something.
I just plan for regular vet visits in advance and have a small savings account set up just for emergencies with the pets or unexpected vet visits that we replenish if needed.
No offence to anyone who likes using insurance, but I look at it this way - put the money you are paying for a monthly premium in a good savings account instead, not only will that money accumulate and be there for needed vet visits, it's collecting some interest instead of lining the pockets of insurance company owners who won't do what they're supposed to for your money. Anyway, lol, that's my rant. I feel bad for you, I'd be on the phone with a manager giving someone an earfull if it was me
I think someone else posted about that same company raising her monthly premium significantly. Can't remember exactly, but even if it was a different co., still shady if you ask me.
Good luck, hopefully they'll pay it for you.