In my eyes you are just experiencing a highly questionable behaviour when trying to extend paths with the Paintbrush tool at an acute angle (roughly said) when the Edit Selected Paths option is turned on in the Paintbrush Tool Preferences.
Unlike you stated, you may already have noticed that extending a path at an acute angle not only adjusts the existing path, no, in fact it deletes the previously drawn path.
One common workaround is to first slightly continue the path at an obtuse angle, then make the turnaround. The existing path will not be deleted in that case. In the video clip you took that way in your second example.
Another workaround is to disable the Edit Selected Path option or the Keep Selected option which both disable the ability to extend the paths at the same time. You would then have to manually join the separate paths afterwards. Sometimes acceptable, often a bit lame.
In earlier versions of Illustrator the Pencil tool had the same weakness, but that has changed. Nowadays it allows you to draw at cute angles lively.