We're going ahead and deprecating bDefaultViewportMouseLock and it's associated bools when changing the input mode of the UI, and switching over to controlling mouse lock with an enum value. bDefaultViewportMouseLock will now map over to only locking the mouse to the viewport while the viewport has mouse capture, and locking the mouse to the viewport always will need to be explicitly stated when setting the UI's input mode.
After clicking once inside of the window the lock mouse to viewport functionality is lost. The mouse can then leave the viewport. This happens in standalone as well as PIE
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-7293 in the post.