Unexpected triangulation results for a concave shape

When triangulating a concave shape, I would expect that the edges of the shape would be preserved such that the outside edges of the geometry would become vertices for 1 or more of the resulting geometries.

If this assumption is incorrect, is there a method or parameter that could force this to be true for the result? Is that easily achievable?

Example plots:

Code to create the same situation (using same below geometry):

triangles = MultiPolygon(triangulate(largest, tolerance=0.001))

Here’s the WKT for this example geometry:

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Shapely-1.6.1 Python 3.6

[dr-jts]

Delaunay Triangulation does not respect edges - it only takes into account points. To respect edges you need to use Constrained or Conforming Delaunay Triangulation. JTS has a Conforming implementation; not sure of the status of this in GEOS.

[mhtb32]

Conforming triangulation has been put on Milestone 3.9 of GEOS. Maybe Shapely can have this after GEOS implements it?