What I expected to happen was: The spider would stay up against the ceiling.
What actually happened was: The spider appeared to drift down and could not be hit.

The spider starts to track the player and climbs up the wall. After the spider climbs up to the ceiling, it will sometimes seem to drift down while frozen, but it is not able to be attacked or moved through an opening in the wall. It will then snap back to its original position against the ceiling, and the process randomly repeats.
This only occurs when the player cannot directly see the spot against the ceiling where the spider is supposed to be.

To reproduce:
1. Build the wall seen in the photo, with a lip (acting as a ceiling) on one side.
2. Stand on the side without the lip.
3. On the other side the doorway, spawn a spider.
4. Attack it to make it track you.
5. When it moves down from the lip, attempt to hit it.