I tried alternating between the availiable actions and it just went back to 'Simis thinks Ella is okay' after the interaction had finished.
I had this happen just recently, too, in my Dynasty household in Barnacle Bay.
When I was trying to get Odessa Cross married, the first candidate was Dude La Mer. After spending all day talking with him, the relationship bar was maxed, but any time I did a romantic interaction, it never went anywhere. It wasn't rejected, though: the status would go from "Dude thinks Odessa is OK" to "Dude thinks Odessa is being Flirty," then
immediately back to "Dude thinks Odessa is OK" the moment the interaction was finished. This went on for several in-game hours.
But, before they had met, Dude had moved in with Joe Pesce, so when Joe got home from work she started talking to him. She was able to pursue a romantic relationship with him and they eventually got married without any issue.
Dude and Odessa both have the "Childish" trait, so I originally assumed it had something to do with that, but perhaps this is a bug.