There seem to be some compatibility issues with Marin's Enhanced Staircases, as shown in the GIF below. I tried it with stairs facing the other direction and the same issue persisted. I also tried it without the tree near the bottom of the stairs.
It seems to have to do with the passability of the sides of the stairs, specifically the passability of the tile directly in front of the player. Normally, they're set up like the image below, and this setup results in the error mentioned above. If I set those impassable tiles to passable, however, the follower behaves as expected, but having the player be able to walk on those tiles isn't ideal.
I tried to gather as much information as possible, let me know if you need any more information or can't replicate the same error if you try to recreate it yourself.