Can insurer rescind Prior Auth after procedure has already been completed?

I have United Healthcare through my employer, I live in Massachusetts. After a few attempts, they gave me Prior Authorization for Botox for Hyperhidrosis (I get a horrible rash from any strong deodorant, and my derm and I have tried lots of other solutions, this is the only thing that helps me). I got the injections, was over the moon, then suddenly a $3000 bill shows up saying it was denied. I’ve appealed twice, the first time they took some of the cost off, but they still want $1800. They said they don’t cover Hyperhidrosis, to which I ask: why did they give me prior authorization?

My main question is – are they legally allowed to do this? I thought the entire point of prior authorization was to avoid surprise huge bills. It seems incredibly unscrupulous that they can just saddle me with a bill because they made a mistake.

Thank you so much for any help or advice you can give, I tried researching if this was legal but I couldn’t find any sort of clear answer so I hoped Reddit might be able to shed a little light