What is the use case for ‘3d Polyline’ entity? (Created via Shift + Freehand tool)
It creates a shape (3d polyline!?) that can only be selected from the endpoint and with ‘window selection’ (left to right) (unlike other selection types). It is indeed a stubborn entity!
The intent is to give you an option for tracing non-straight edges without inferencing being active. It would be useful for tracing things like contour lines on a topographic map or other very irregular curves.
After it is drawn, you can select it and explode it from the Edit menu. If you don’t want it to behave as a collection of unrelated edge segments, weld the segments together.