While this isn't what usually happens, it isn't a bug, it's how their walking AI works. They walk wherever available within a certain range, even through doors.

When you open a door, the clipping code is temporarily updated, so that npcs and other players can follow you through it. In theory, you're right, npcs shouldn't go through there. On the other hand, I like the mechanic how it is. We'll just leave this for now, because it's not causing any harm. Thanks for your report.