Non-solid, Horizontal Mechanized Airlock

Pictured here is a sideways mechanized airlock that lets gas freely pass through it. Gas does not get trapped within it, gas just flows through as if it wasn't there. Setting it to open and then to auto fixed this. The settings of the pictured door is automatic, but set to not let dupes in from either side (the door's purpose was to keep a pocket of chlorine contained using the gas's weight alone). Dupes can't run over the door, so it appears to be totally non-solid.

This all happened while I wasn't looking, so I'm unaware of a method for duplicating this. Additionally unfortunate is that this happened at some point in a period of about 20 cycles and I've lost autosaves to check when it started. When loading an autosave made during the "fake door"'s existence will show gas inside the door, but unpausing will release the gas, which is what you'd expect when gas is inside a solid. This shows that the door is solid again upon loading a save.

I wish I had more info, I can imagine this thread is hardly useful.