Hello, welcome to the forum!

It's a big help if you could please post the .3dm model file along with your question. That makes it possible to examine your model in detail, zoom in on pieces and test stuff out which can't really be done just by looking at a screenshot.

Just looking at your screenshot my best guess would be that once you have added the triangle the juncture at this location here is difficult for MoI's filleter to resolve:


That's probably something like a "multiple colliding fillets of different convexity" type of thing. The fillet engine in the geometry library that MoI uses just has not had the hundreds of man-years of work invested in it to recognize all these different kinds of corner juncture cases. There are other CAD programs like OnShape for example that could handle that one probably though.

- Michael