Manually adjusting white line in subject isolation on iPhone

How to manually move part of white line around defining subject, but doesn’t allow line to completely encircle subject. White line on a portion of subject just traces to outside photo edge.

I am trying to isolate subject in foreground, so I tap on subject and white line appears, however it only appears around part of animal and then outlines to edge of photo, thus including part of room. Can I manipulate the white line?

[Re-Titled by Moderator]