Amazingly, I think I kind of run this kind of thing like
@Reynard is describing - though I have no hard and fast rules about it, it is a case by case basis. I think the question is, is the character trying to recall information based on objective fact divorced from their experience of hearing stories about the creature or even studying it or are they recalling what they might know, whether or not it is 100% factual.
I have a general rule in my current games where PCs know whatever the players think they know about a monster (characters by design come from a place where most monsters are extinct or sometimes, if possible, kept in a zoo or a monarch's private collection). I am just never in the mood to police what players think their characters know or don't know about stuff like that.
So if you think that trolls are vulnerable to fire, you are welcome to think that and do what you will based on that info. You probably don't know about this specific variant of troll that uses its regenerative ability to perform body modification, implanting countless stones in the flesh of their back - as a reaction they can turn their back to the source of fire damage from a targeted attack and only take half fire damage.
Recently, the PCs had an encounter with a vampire. One of the PCs wanted to make a check to know if they had ever heard of vampires who could curb their appetites and be neutral or even try to be good (which the vamp was claiming).* Despite having a successful enough roll to learn a lot of about what vampires can do, it was not high enough to determine that yes, in theory some vampires can make efforts to be less evil. So the answer had to be, "No, you've never heard stories of good vampires, but you have heard stories of vampires pretending to be good." The thing was, the vamp was sincerely trying to change his ways!
Ultimately, the player decided that just because all the stories they'd heard said X, doesn't mean that Y could not also be true and risked a negotiation with the vampire. What did he base this choice on? Other information the party had gathered about the vampire's recent activities and behavior which shed more positive light.
My point being that when you move away from binary yes/no results regarding info like this, some successes might still come with absent or even erroneous, or more complexly, "partially true" information.
* like the vast majority of monsters in my games, vamps are homebrewed.