When I start TS2019 it always begins in full screen mode and there is no obvious way to change to managed screen size.That was true until I just started to try anything, ignoring logic. What I found was, while in full screen, on the opening screens, the top left 3 horizontal line symbol allowed starting the Launcher. Clicking that restored full control of screen size. No problems from this technique. I would guess that a full re-install would be a final cure. Too dangerous to consider.
