Stepladder turned into a floor

A stairway to an upper floor in the safehouse I was using got trashed by zombies, so I bashed it out and replaced it with a stepladder.

However, after saving and loading back into the game, the space above the stepladder has transformed inexplicably into a ‘flat roof’ tile, making it inaccessible and removing the only way down in this house. :confused:

Anyway, climbed down the side of the building and removed the ladder from below to set it up elsewhere. The weird new ceiling/floor tile persisted.