Hi Alain - thanks - I see the problem here as well - It looks like it is a matter of setting the radius small enough to be OK with the end radius:
But it looks like the fillet needs to be even smaller, in this case, than the expected minimum - I’m still looking at this…
@Alain1, please try this:
Explode the original object.
Select the vertical surface,
_InsertKnot, in the V direction (in this case the vertical direction) and use the Automatic option maybe two or three times.
Join the three surfaces back into one object and
_FilletEdge again, with a radius of .75 or smaller.
This seems like a bug in Rhino 5… V6/WIP gets the correct answer for the original object.