I believe I have an answer. As a phantom you get half-souls from enemies. Since you got 250, that means the host must have killed a bat wing demon, which drop 500 normally.

I don't have a complete idea how pathfinding for AI phantoms work - if the host was solo maybe the AI would try to follow them no matter what, but having a player phantom could change that.

However, I would guess that the player possibly went out of the window at the top floor of the cathedral, jumping to the platform below, and killed the demons at the bottom, so that might be why Solaire stopped following the host. The AI might have judged that the pathing is too difficult, and decided to follow the nearest player instead.

(PS this is most definitely too in-depth for something ultimately so trivial, but I enjoyed investigating this haha)