Thanks for getting in touch.
Only the curve underneath your finger will "snap" to create connections with other curves on the page. This gives you some control over when new connections are established....
We are improving this in the next major update. You will be able to control whether snapping happens with this new button.
And as far as "sticking" goes, you can use the "shake & break" feature to break the connection.
In your example of two grouped rectangles, start moving one rectangle and then shake it back and forth quickly while doing so to detach it from the other rectangle.