I would consider it reasonable on the fly, but ultimately I would change it for next time.
I understand from the player's POV they need to know in case a later attack hits and if they want to spend the ki to attempt to stun again. For them it is a resource management issue. Ki is limited enough IME and players shouldn't have to be forced to use it again because narratively you feel they would not be aware if they "restunned" the target. There are more than enough ways to narratively justify they would know, after all this is their KI we talking about hear.
Let me ask you this: if a caster casts Charm Monster on a creature which is smart enough to understand what is happening, and it makes the save, but pretended to be charmed, does the caster know the creature is actually charmed? A target with a great Deception check could easily make the caster think it was still charmed, couldn't it?
If casters know if their targets succeed or fail, so should the monk IMO.