I understand if he's super undetectable to even surpass dragon sense, but that skill grant invisibility & I(invincibility)-frames? How is he just laying in the fire like it was still a grass hill? Tanks melt in the fire & he's just chilling as a sniper. That's some BS.
My guess is that it is an error in the adaptation. From a distance, such as the cousin's perspective, it would have been unclear if the flamestrike hit Leeha, but the AoE is so large it would be easier to believe it hit than it missing. What it probably should have looked like, once it changed to Leeha's perspective, is that it hit close, but not on top of him. Maybe the original described it as "everything around him being burnt" or something to that effect. But yeah, as portrayed here, he should not be fine just because of the camouflage. (I, myself, was wondering if his barrier ring could withstand the flames, but it doesn't look like he used that, and it would probably give away his position anyway.)
To be clear though, I'm purely guessing. I haven't read the original. I just know that these kinds of errors show up in adaptations like this.