No idea. It was an issue in Skyrim as well. My guess(again, just a guess) would be that the scripting to take you into that cell is linked to an individual door opening animation and when an NPC uses it, you have to reset it before using it. Since 4 and Skyrim use pretty much the same engine(I know, they used the same engine for most of their games, but the versions of Skyrim and 4 seem pretty similar) so they share similar issues.
That's just my guess. Maybe someone else has a better answer.