Joining symbolic lines at corners

[Anonymous]

Why is there a difference between using a fill and a polyline, in how they join at the corners when using a symbolic linetype? Surely they they should work the same way?

The polyline (on the right) automatically joins, but the fill (on the left) doesn't.

Is this an error, or is there some kind of twisted logic to it?

EDIT - Quick update. Roofs, Slabs, Meshs and Polylines all join correctly, but Fills, Lines and Poly-walls do not.