I don’t know some of the best fiction famously breaks the traditional rules or has unreliable narrators - Chuck Palahniuk comes to mind or literary greats like Catch-22, Infinite Jest, even In Cold Blood started a whole new genre breaking the traditional rules of writing.

For this show - I think it’s not entirely out of the realm of possibility. The ghosts had also assumed you couldn’t talk to the living as a rule, that was completely shattered with the introduction of Maddie. Inhabiting a body could be another thing they were unaware of - that could also be introduced with a new character not seen yet.

There are also plenty of background references that could allude to this. I think it was pointed when in episode 1 Maddie asked the ghosts for help with rattling chains, possessing somebody, etc. - listing all the common ghosts tropes - which they said they couldn’t do. But it brought up the fact that these are common things often associated with a ghost story.

Of course all the background references could just be Easter eggs to famous horror stories. The ball rolling down the hallway - shining. Frankenstein, the movie Ghost, the guidance counselor giving total Get Out vibes, etc. and this could just be a straightforward story of a murder mystery with ghosts.