I have no idea, good question. I'd suspect that since the terrarium is in "his room", and was his thing/action, that when the new spouse's room is formed it contains their stuff only, so I'd expect that the terrarium is gone. I look forward to hearing the answer and/or confirmation with you though, to see if it functions as I expect or not.
Since CA
could do whatever he wants with this, he could have left it maybe. It is this kind of stuff I love to learn about the game, to see if it does the expected thing or does the quirky thing (there's no rule that it can't, which is always fun)!