(See the best response by Ter13.)
When I'm on one tile, the building shows, like in the pic below.

When I move one tile to the right, the building disappears, like in the pic below.

Would anyone know what's causing this, and how would I go about fixing it? This has been happening ever since I changed the map format from TILED to TOPDOWN.

That's normal. Your buildings are too large. Try setting the bound_width and bound_height to the pixel size of the building. If that doesn't fix it, you need to break the buildings up into 4x4 tile chunks.
I thought bound_width and bound_height are only for moveable objects. My buildings are turfs
Best response
turfs should (almost) never be larger than icon_widthxicon_height.

Change them to objects and use bound_width/height to try to fix the overflow.

If they are turfs you can't fix this except by breaking them up into 1 tile by 1 tile individual turfs
I see. Thank you, Ter.