I can not duplicate that.
I drew a single span degree 3 curve with 4 points.
Turned on the points, selected an interior point and deleted it.
The curve changed shape and had 3 points.
I selected the last interior point and deleted it.
Now the curve is straight with just end points.
I have noticed it too. It is not regular behaviour and does not have a pattern.
It happend for me several times, last one with curves created with Elk plugin from GH.
Example file attached, See crv marked with Revision Cloud.OSM.3dm (1.0 MB)
That’s a degree 1 polycurve, but it acts the same way.
If I select the polycurve and run PON, then I can select the inline control points between sharp angles and delete them.
If I select a point on a kink, it changes the shape of the polycurve.
Is there any change you are selecting the curve itself (not points) and tapping Delete?
Then it would act just like you described.