Not even about how this isn't a greenhouse, but the game doesn't qualify this room as a room even though the only qualification for a room is to have it enclosed by tiles and/or doors.
I have experienced this bug multiple times when the rooms have levels inside even though that should be allowed still.
Apparently, only some parts of the room are not treated as a room as the second screenshot shows.
Create an enclosed room with levels, see how it doesn't get treated as a room in the room overlay. Try to play around with the room and find that only some part of the room doesn't qualify to be a room.