Strange bug where if a shaft is immediately south of a half dirt slab that has grown grass, with a solid block on the other side of the slab (ie north), the north face of the shaft is transparent.
I can only reproduce with grass half slabs, no other I tried.
the side towards the half slab doesn't render. only the side facing north
test it with a full grass block near a half grass block and shaft. if you remove the full block it renders properly