Good result - except the top surface is missing.
FilletSrfMissingSurface.3dm (232.6 KB)
Work In Progress
(9.0.25154.12305, 2025-06-03)
Added: The shape above does not have a back surface. I added a back surface. With that FilletSrf does not extend the fillet and trim the top surface. Problem in both situations, with and without back, appears to be caused by fillet not extending.
FilletSrfMissingSurface2.3dm (377.4 KB)