But if it becomes a pattern, there's very likely more to it: they're reading the MM (a neutral act in and of itself) and acting on what they've read (thus playing in bad faith).
Wouldn't it only be playing in bad faith if the table rules dictated it as such? I mean, it's a pattern in my games, as I've pointed out, I encourage my players to read the books and manuals, to determine what their characters may or may not know. If they decide their character has knowledge about a specific type of monster after doing so, they are following my instructions, no? How is that bad faith?
Most of my players aren't reading the MM or VGtM, since they really don't have any interest. But a couple of them are DMs so they know more about them, and we also have discussions about ideas for games where we aren't players. So I know that they know a lot more, and sometimes that info enters the game, sometimes it doesn't. But again, my assumption is that if you live in a world where monsters are real, you'll pay attention to the lore you hear, even if it's about a monster you've never seen.
The problem only exists if there is a rule that prohibits it. As I've pointed out, the character is really only capable of acting out of player knowledge. It is, after all, a fictional construct. Other than rules to allow the character to have independent knowledge (skill checks, or the DM filling in blanks the player may not know), they can only act upon what the player knows.
The issue has nothing to do with whether the character is acting upon player knowledge. It is entirely a question of what player knowledge a given table excludes.
For example, some have indicated that they allow other players to make suggestions, even when their character isn't present. I myself often allow this, because the game is entirely in our heads, and the sort of decisions and observations a character living in the world with full access to all of their senses is quite different than what goes on in our imagination. So sometimes some extra help is welcome. I actually prefer that to prodding or suggestions from me (the DM) because anything I say takes on a greater importance. "Oh, if the DM points that out, it must be a big deal."
In addition, based on my experience playing, running, and watching D&D and other RPG games, there's a lot more metagaming than people usually give themselves credit. You can't have the game without it. But it's only certain circumstances where it triggers their metagame alert.