This should work. Maybe I am asking too much for the year 2021? Should I come back in 50 years when humanity has figured out how to tell computers to fillet a curve?
All jokes aside this is infuriating. This is basic stuff, and I know the fillet in GH has been crap for years now. What is it, 50 lines of code?
Look at the segments of the curve. If you increase the radius from 1.0 you see that it stops working on that corner as soon as the fillet touches the endpoint of the segment. One way to fix this is by joining the segments between discontinuities and rebuilding those segments. You can rebuild this closed curve with three segments.
Your curves need to be split at discontinuities first. Join the segments between discontinuities, then rebuild. It works with my definition posted above.
Then we have the ever existing problem of curves not being filleted because of the fillet Radius is too big… oh well, by hand it is. Will come back in 50 years.