MergeFace fails but why?

Hi,
V5

why does MergeFace fail on this joined surface ?
Mergeface fail why.3dm (67.8 KB)

Baffled

Steve

For what it’s worth: MergeAllFaces Does work on your example.
There is a tiny surface strip on your polysurface that seems to cause the problem when just trying to merge the two main surfaces.