Hi Pilou, it is a bug but like many fillet bugs it will not be easy to fix anytime too soon.

On some closer examination, it looks like the big problem here is that there is a very small edge right in that area:



Basically this has a situation where there are 2 corners separated by a small distance, instead of one single shared corner. That type of small edge creates a very small compressed area which really will tend to mess up fillets. If those were more exactly aligned it would have a better chance. But the best idea is to get the whole seam edge out of that area if possible as you have shown by rotation.

The fewer edges involved for fillet, the better chance it has.

My guess is that somehow the placement of either the revolve or the cutting piece is just not quite lined up with each other perfectly. This is the kind of thing where a very accurate and snapped together placement can help to avoid fillet problems.

- Michael