Why did the Workbench mesher not mesh a narrow 180 degree
toroidal connection between two solid cylinders? It resulted in the
two parts of the model no longer being connected.
This is a special case of a torus with a major diameter
that is on the same order as the element size. The flat
surfaces where the torus meets the cylinder are too close
to each other and the mesh jumps across.
Development reviewed the error and determined there was no fix
available without a major change to the algorithm. To catch this
geometry by default would slow the mesher down significantly. Reducing
the element size or increasing the relevance corrects the problem.