Issue #29464: Fort Greymoor Prison navmesh
Has 4 islands, each overlapping a vertex with the main navmesh. Bad autogeneration.
One is the main hallway, stranding the bandit dissecting the troll. Also prevent one of the xMarker patrols from working. And of course, prevents your follower from following (which is how I'd noticed).
I've painfully merged (took 4 tries) the main hallway into a single navmesh. Reduced those two islands to single triangles (deleting one by one, from highest to lowest). Z'd them up in the air, but not too high.
The other two islands were each already single triangle, one of them under some cages. leaving half the room with no navmesh. Z'd them up in the air, and added navmesh for that half of the room.
My follower now follows.