if the edge in group is touching the boundary of the geo, the boundary point is not split by default
so you have to check this option
like when you do a grid and select one row in the middle to cusp, you will see the difference
to see it more you can move primitives of one cusped half and you should see it not fully split unless this option is checked
it's usually more about near clipping, but overall both are important
so make near clipping lower, but still make sure far is not overly large
