Opposite side of trail spawner dissapears when block gets placed next to it.

What I expected to happen was...:
when block is placed next to trail spawner, that side dissapears.

What actually happened was...:
I placed block next to trail spawner and the opposite side dissapeared.

Steps to Reproduce:
1. place block next to trail spawner (works only on one of the sides on trail spawner)

I think it is north side