Hi Pilou - that is actually working normally.

By selecting just the top loop you are telling MoI that you do not want to fillet the side pieces and that those should remain sharp.

To keep those edges sharp means that the fillets will have to be constructed in the way that you show - the only way to make them rounded in that area would be to also round that side edge as well.

The only other alternative would be to open up a gap in the model, like this:



The area shaded in red there would be a hole in the model.

It isn't possible to be simultaneously sharp and rounded and connected together all at the same time...

- Michael