Presumably, it's a generator bug causing terrain displacement. I suggest adding the following point to the rules:
Displaced Terrain. If an unguarded passage is found between zones and there is no visible guarded passage, allow free movement into that zone.
Justification: In the game, it’s not always possible to determine whether this is a bug or not. It may be that this passage was created by the opponent earlier. You wouldn't ask him about it, would you? There aren't always judges or moderators online who can quickly receive the save file. So, it's better not to hinder the gameplay, in my opinion.