Ahn, yes forgot that he can be in your party at that point. While he was not going to stay in your party for long anyway it's not an intentional thing that he'd vanish there! Thank you for finding that one, I am expecting many more things like these, to be honest. I'll see what can be done~
I should probably have set the comments up as a forum now I think about it. For now the best way would probably be the thread at RPG maker central https://www.rpgmakercentral.com/topic/21816-corruption-of-laetitia/ I will see if I can find a better way.