Unsubdivide doesn't work Blender 2.80

A number of steps (days) ago I accidentially subdived my low poly model. When I now try to undo or at least reduce the number of vertices with Un-Subdivide simply nothing happens. Un-Subdivide has absolutely no effect.

Verifying the issue with a new cube in the same file Un-Subdivide works.

The documentation says nothing about any limitations.

So is there a way to force unsubdivide the mesh.
The model is a partial adapted model from a larger one, made with triangles, already with vertex-groups and weight paint. The tris seems to be gone after the subdivision.

For the moment I'll rebuild the low poly model from scratch but I'm curious how I should proceed if a rebuild is not acceptable.
According to Campbell Barton Un-subdivide currently requires grid topology, which means it likely won't work on a triangulated mesh. The discussion on the issue tracker can be found in T65748.