This is a regression from Studio driver version 462.59 to 471.11 (and continuing to current 472.12).
In 462.59, when an Ansel session was started (the UI came up), it would hide the windows cursor and show its own cursor captive to the game window. If you Alt+Tabbed out or used the Windows key to wrestle focus away from the game window, Ansel would hide its own cursor and ignore keybaord and mouse input.
In 471.11 and after, when an Ansel session is started, it still hides the windows cursor and shows its own cursor captive to the game window (same as before). However, now when removing focus from the game window, the Ansel cursor stays active and responds to all mouse and keyboard input, as well as the actual windows mouse. This leads to clicks on other monitors or keyboard strokes in other focused windows actually being responded to by the Ansel UI as well.
This causes issues for us as we use Ansel to render images in the background while continuing to use the workstation to perform other tasks.
This occurs in both UE4.26 and UE4.27 and is repoducible only in the driver versions stated above. Additionally, this is using the original Ansel implementation (in the Program Files\NVIDIA Corporation\Ansel\ folder), since the new Ansel implementation does not allow rendering in non-fullscreen mode, and is unable to perform a background render when the game is not in focus.