But no need to used (for avoid any problem) it if you turn the Generator curve of the sphere before the Union! ;)
It must don't touch the cube!
Then Fillets works like a charm! :)



Try with this one! https://moiscript.weebly.com/uploads/3/9/3/8/3938813/exo0905.3dm

Don't ask me why! :) you must turn it! (fillet 1 will be fine)