This occurs in 7.0.20309.6003, 11/4/2020, but not in V6:
- Add _Curve of any valid Degree (1-11).
- _InsertKink anywhere on the interior of the curve.
- _RemoveKnot any amount of knots from the added kink.
Per _List, curve is now rational with weights at end CPs at 1.0, but some of the other CPs have weights within 1e-12 of 1.0.
For a degree 1 curve, when only CPs at the end remain, _List reports the weights as “1”, but “is_rat = 1”.
This doesn’t appear to occur with surfaces.