Anyone ever seen this happen?
The images speak for themselves. Any radius between 2.9mm and 4mm has this result, anything over 4 fails, and anything under 2.9 behaves as one would expect solidworks to behave.
This is the preview of the fillet that also shows the part pre-fillet.
The front face is curved in two directions, and the top face is flat. The side is an arc.
And this is after the fillet command.
I'm surprised the bottom didn't fillet the same way.