Yeah. It's insane how many times this gets posted and people have to put that tired old George quote where he explains this.

The Targs aren't fireproof, Dany isn't fireproof, what she did was a one time, unexplained, miraculous thing.

Targs have SLIGHTLY increased resistance to heat, as demonstrated by the scalding bath and Dany touching hot objects, they are NOT fireproof, Viserys died with molten gold on his head, one Targ tried drinking wildfire trying to prove this, and others were burned alive during the dance of the dragons and blackfire rebellions.

In the books it's blood magic, and likely Quathie is responsible. The show decided never to address that character again, but I suspect she would be responsible for the second time in Vaes Dothrak if that happens in the books as well.

For all we know, Quathie may also do something during this battle in the books too to explain why she may continue to have fire immunity, but it's the fact they abandoned all that potential plot to say "nah, she's just fireproof and we're never going to explain why" that's the problem.