I went back and found the post where Isen found Wilhelm, and I can see where the confusion came up. It looks a little ambiguous, and it looks like I made a different assumption than you did. Isen was on his way back toward Mount Beauvray from exploring the regio side, and I thought that he was still on the regio side and that Wilhelm had somehow stumbled through, while it looks like you're saying that Isen had crossed back into the mundane side when he found Wilhelm.
But like I said, I'm cool either way. I'd be cooler with him being on the regio side, but shrug.