Another find, trying the TriEd mode. The collapse code in UEditMeshPolygonsTool::ApplyCollapseSingleEdge() there looks right when glancing at it, but it appears it somehow actually collapses a different edge than the one you selected. Maybe its a “random” neighbor vertex/edge ID that sometimes just doesn’t exist, which could explain why it sometimes does something and sometimes nothing at all.
Collapsing the selected edge in the screenshot actually collapses the one marked in purple:
=> I will consider Modeling Mode not ready for productive use for now and stick to Blender 