The Algae Terrarium empty animation seems to only play with the dupe on the right side of the terrarium. This happens even if there is a wall on the right of it, in which case the dupe will appear to go inside the wall and proceed to empty the terrarium from inside the wall:
Note that the dupe is standing in front of the terrarium while emptying it, and it's just the animation that plays that makes it appear like he does it from inside the wall.
Steps to Reproduce
Build an Algae Terrarium on the left of a wall or a wall on the right of an Algae Terrarium. When the terrarium is full, the dupe emptying it will play its animation as though they're emptying the terrarium from the right side of it, which places them right inside the wall.
This issue has not been confirmed by a developer yet.