seems so simple, but...

 From:  Michael Gibson
5400.12 In reply to 5400.11 
Hi Mauro, booleans are generally a strong area in the geometry library that MoI uses, but fillets not so much. Fillets basically have a larger chain of complex operations that they need to perform and so correspondingly there is more that can go wrong.

Basic fillets work pretty well in MoI but things that require special case handling (like disappearing fillet is one example) tend to be problematic.

- Michael