Hi marc, so the rounded corners on borders is something that normally happens in any polygon modeling program when applying subdivision.

You should see the same result in Cinema4D if you were to apply subdivision smoothing inside there like this:





So the rounding that you see in MoI is just trying to behave the same as polygon modeling programs do when performing subdivision smoothing.

It is better in general to avoid having large open borders though because the behavior changes a little bit in those areas and there can be only G1/Tangent continuity between pieces in those border areas instead of the regular G2/Curvature continuity. When you have a closed mesh it helps make totally uniform higher level continuity behavior throughout.

It could be possible in the future to put in an option to make MoI's subdivision at borders be clamped down like the Fusion360 result that you show. But that would be different behavior than what you would normally see in any polygon modeling program.

- Michael