Metalevel considerations on this are hocus bombus. Questioning the nature of perception is a slippery slope. We don't remove AC bonuses from fighters on the basis of whether they percieve that a shield was necessary, or remove evasion bonuses from rogues on the basis of whether they percieve that they triggered a trap or a wizard was casting a spell. It is the responsibility of the DM to provide subtetly beyond a static label of "You are frightened or charmed or under an illusory effect."
Just don't tell the players any more than the character knows. Let them make up their own minds and roleplay as best they can. If the player doesn't use the ability and it bites them in the ass, so be it. Or vice versa, don't railroad the table because you can't deal with players using their class abilities to play the game.
Further, monks are trained to engage a disciplined mindset of rational consideration. Anything that impedes that with violation of their martial axioms and unwelcome emotions violates stillness of mind.
Even further, in terms of practical consideration, how exactly do you use the class ability if the effect it specifies circumvents usage? Why would anyone write such a useless runaround, unless the intent was to provide a charm counter?
In which case, yes, the monk can know they are charmed, and can end the charm by using the turn to mantra or littany or whatever.