Holly & Mr. Whatsit

So in episode 1, Mike knows about Mr. Whatsit, knows that Holly has an “imaginary friend”. And he doesn’t at all think about the possibility that Vecna is playing mind games with Holly, after everything they’ve been through. After people in the group have stated multiple times that Vecna’s play is to get into your head. So why didn’t he ever bring it up to the party, or have El check Holly out to make sure she’s okay? Kids have great imagination, but with everything going on, you can’t overlook anything.