Some buildings can be built through tiles (not diagonal access, but actually through an entire row of tiles). This appears to only happen when building from ABOVE.
It seems to affect doors, minipumps (in any of the 3 rotated positions), conveyor loaders and receptacles, valves, shutoffs, etc. Above, the conveyor loaders appear to both be buildable, but only the rotated one is actually buildable.
They can build the conveyor loader, but cannot put materials in it from above (so at least that isn't a bug).
Ask the dupes to build something, from above. Watch them build it. Start with a door, then move on to lots of other rotatable items.