Hi PaQ, yeah what seems to be happening there is filleting is running a face merging process in one of its ending steps, and it's merging those 2 sliced fillet faces together into one single face. Then as a consequence of the merging one style is lost since you can only have one style per face.

I could look into turning off the merging step, but it's probably not advisable to tweak something like that right before a release so I'll need to look into it for v4.

In the meantime, the reason why those pieces will get merged is because they share the exact same underlying surface since it looks like they were cut from an initially larger fillet. If you run ShrinkTrimmedSrf on the object before doing the fillet, that will shrink down the 2 underlying surfaces for those pieces and then they won't merge together after the fillet.

- Michael