Hi Stien, in addition to OnShape you could also try bringing your model into ViaCAD as well, the fillet engine that either of those use is able to handle these types of fillets that meet up in sharp corners quite a bit better than MoI. MoI's filleter will probably work fine for the case of all the edges being filleted like in my above screenshot.

The type that you're trying to do just doesn't work well in MoI right now unfortunately.

- Michael