Hi friend, yes filleting is complicated. For your case there I don't think there is sufficient space to fit a natural fillet in the corner spots much larger than about 1 unit or so before it starts to bunch up on itself:








There is some stuff that tries to deal with some kinds of fillets where there isn't enough space for them to fit but it's harder for that mechanism to work when the bunched up area is pretty small. That's why you're able to get some larger radius values in there.

In general it's better to avoid trying to place a fillet in a spot where there is not enough room for it to exist without bunching up on itself.

For your model here the way you would do that would be to not put in the rounded corners in your lofted object and instead have them sharp and round all that off in one single fillet like this:





That way these areas become corner juncture patches generated between fillet surfaces:



In yours instead of the filleter constructing a corner juncture patch it's trying to build a fillet surface between these pieces:



If you start with the sharp cornered shape here you should see a smooth progression of fillets for increasing radius values until they get large enough to run into each other much larger than radius=4:



- Michael