GRRM did say that it was a one time event however it does not seem to have held up. In the books as well, Daenerys has shown ability to be (almost) unscathed by fire more than once. First when she gave birth to her dragons as you mentioned. Second when she claimed Drogon at Draznak's pit as Ser Barristan recollects:
Her hair was aflame. She had the whip in her hand and she was
shouting, then she was on the dragon’s back, flying.
We know that part is true because Chapter sixty three of Dance with dragon says:
She ran a hand across her stubbly scalp where her hair had burned
away, and felt more ants on her head, and one crawling down the back
of her neck. She knocked them off and crushed them under her bare
feet. There were so many ...
And as we have Dany's own account as well:
With a hiss, he spat black fire down at her. Dany darted
underneath the flames, swinging the whip and shouting, “No, no, no.
Get DOWN!” His answering roar was full of fear and fury, full of pain.
However, counter argument here would be that Daenerys tried to get away from that fire so she escaped with no more than her hair burned which is unlike the first event when she went into a pyre completely. But I suppose it is a weak argument because come on, it's dragon fire. Daenerys must have makings of a great athlete if she can be that quick.
But we do know that Targaryens are not fireproof, this is something that appears to be unique to Daenerys.
- Viserys Targaryen died of being burnt by molten gold.
- Rhaenys Targaryen was burnt to death by Vhagar and Sunfyre. (We can't be sure though that if it was the fire that killed her or the fall. But we do know her corpse was so badly burnt that it could not be recognized and that she was indeed dead).
- Rhaenyra Targaryen was burnt by Sunfyre.
- Aegon II was paralyzed due to injuries given by fire of Rhaenys Targaryen's dragon Meleys the Red Queen.
- Aegon V was burnt with Prince Duncan Targaryen at Tragedy of Summerhall. That left House Targaryen family tree limited to one lonely branch of Jaehaerys II.
- Aerion Brightflame died after drinking Wildfire. I suppose we can call it internal chemical burning.
- All deceased Targaryens are burnt in a funeral pyre which further shows that their bodies are not fire proof, as pointed out by Arnaud.D. David has provided a good counter-argument for that. He suggests that this supposed immunity could be tied to life-force of Targs (Even though that is disproved by numerous occasions of Targs dying due to burning).
This shows that Daenerys did continue to display her resilience against fire in future events of books as well. As you have noted as well, Show is heading into a completely different direction. How did Daenerys manage to appear unburnt out of the cottage? I fear the only speculation we can make is that Show runners are going to build on her supposed immunity to fire even further.
We can't have solid arguments in this regard because GRRM has never given us any details as to what exactly made Dany fireproof during birth of her dragons so we can't find similar patterns in future events she might show knack for being fireproof.