Hi steve - so for a completely round cylinder like shape like that it is better to use Revolve to construct it rather than sweeping around a circle path.

Revolve makes extremely precise geometry - Sweep on the other hand goes through a kind of refinement fitting process and on closed shapes it has a tendency I think to have the closing point of the sweep surface to be slightly off from tangent by like 1 or 2 degrees - and that kind of slight shallow tangent deviation tends to be one of the things that the filleter does not like to deal with, so that's most likely what you are running into here.

If you would instead have constructed this one with revolve you would not likely run into the same problem.

The revolve is not only more precise for a shape like that but also has a lighter surface control point count as well.

- Michael