We currently do not have a method in which a user is able to convert an enclosed Path into an 'open' Path.
Or, in other words, we do not provide a method in which a user is able to break up an enclosed shape by deleting a connecting Segment from an enclosed path.

I think that generally speaking, we have 2 different approaches we could take to resolve this:

1) We have the benefit of allowing Segment selection in Studio, so we could simply allow a user to select, then delete a Segment without auto-joining the open Path Points after its been completed.

2) We can follow the Adobe Illustrator Method, and allow the user to break up a Segment by placing a Path Point along it, then deleting the Path Point to remove the Segment. Similarly to Option 1), we would likely need to not auto-join the newly-open Path Points after this has been completed.

Or possibly both.

However, unfortunately, it seems like we should do away with the Path Point auto-join functionality that currently exists, because as handy as it is in some cases, it currently gets in the way of having total control over the Path presentation and prevents operations like the one shown here. Most Designers are used to having to join their Path Points manually to enclose a shape, as this is what the behaviour is in most vector packages, so this shouldn't be too painful of a loss.

See attached video showing the repro steps in Studio, as well as a video showing how a similar operation would be performed in Illustrator.