When entering notes by keyboard pressing s to activate and shift-s to deactivate slurs is working fine.
Clicking with the mouse to activate it works as well, but clicking again to deactivate it does not work.
That’s never been a valid way to input slurs in Dorico, as far as I know. If you want to use the mouse, click and hold to start the slur, drag the slur to where you want it to end, then release the mouse.
Yes, clicking the slurs button always starts a new slur, so that you can do nested slurs (e.g. articulating vs. phrasing slurs).