Pathfinding static navmesh wonky at Orville Airport

[animats]

Orville Airport, Orville sim. A LL-built seaplane airport. I do some animesh NPC testing at this seldom used place. They've been behaving strangely there, detecting invisible obstacles and unable to cross certain lines.

This is an old Linden sim, all prim, little if any mesh. The grey area is completely flat. It's a few big prims. The boarding stairs are prim, about 50 (!) prims in a linkset. Almost everything in this sim is tagged "static obstacle" for pathfinding purposes, so it's good for pathfinding testing. Pathfinding can get a character up those steps into the terminal, all the way to the control tower. But moving around in the open space in front of the terminal gets weird.

This view shows the navmesh for "Type A" characters, avatar-like. Notice the bumps in the flat ground. That's a bug. Somehow, those steps messed up the navmesh for a considerable distance around the steps.

 

Static path viewed in the viewer. Note the up and down movement. The cement surface is flat, so that's wrong.

Somehow, those boarding stairs mess up the navmesh for tens of meters in all directions. This is the only place I've seen this. Any insight on what makes this place special?

(I'm working on some path planning which uses both llGetStaticPath and llCastRay. In this place, they disagree on where the ground is, causing some problems. I need to know if this is a widespread problem, or an unusual special case.)