On the surface, this wouldn't be out of character for the MC, because his first interaction with Liana was also letting a bad guy escape. But that was to show how indifferent he was to anyone else / their situations (after however many years in the dungeon) that he didn't really wanna be bothered, where this feels like him actively involving himself in someone's problems, which seems like a quick leap in character.
Also, I hope him just taking the full-force flame attack here is shown to be him healing through it, because him letting the lizard guy at the start cut off his legs when it was B rank (I think?) was actually a bit neat and played into him being a healer. But if he's entered the zone of "OP isekai MC that just doesn't take damage from A+ ranks in the first place", it'll feel like this story's just taken another step into genericness.