The problem is from Ange's POV (or illusion), until the moment Ange denied Sakutarou's existence (inside the classroom), Maria still believe in Sakutarou's existence . However, the next time Ange saw Sakutarou (on the boat), Sakutarou have already be denied (believed to be dead) by Maria. That's strange, isn't it?
Btw, a question, which's kind of related to Umineko. Not specifically to this episode, but this arc....