In the first episode of tng, Wesley and Data explain that the “simpler” parts of the holodeck scenery is actually replicated, not just projected. I think this was primarily meant to explain how Wesley could walk into the corridors still dripping wet.

But that means at least the water and rocks are real, so when they “end program” it has to reclaim the replicated bits. And that’s how we ended up with “if you don’t shut down the holodeck safely, it reclaims everyone still inside” in one of the early episodes