In the JS API 4.6, when attempting to add a point to the end of a ring or path of a Polygon or Polyline, the insertPoint function seems to do nothing.
In 4.5 this works as expected.
Looking into the source, it seems that the validation is failing when adding points to the and of the point arrays (ring/path). If I override the validation function, it can get past this, but then there are other problems with the graphic not being updated correctly.
This is a bug in 4.6. The problem is that, for version 4.6, the insertPoint doesn't work when adding a point at the end of the ring. We plan to fix this for version 4.7. Thank you for reporting.