I had a failure with a Python script due to what seemed to be a JoinSurfaces problem. But I am now convinced it was a AddRevSrf failure that has been fixed in the latest Rhino6 WIP. With this command now working and creating a closed, single surface, I had to remove the ExplodePolySurfaces & JoinSurfaces commands I was using to fix its failure. Now all seems to be working again.
Previously it seems like this was a BooleanUnion problem. But that traced back to JoinSurfaces returning None. Which traced back to AddRevSrf being fixed so the JoinSurfaces step was not needed.