Under X11, when the user drag the preview pane, he is able to drag beyond the border of the window and even beyond the border of the screen. In the last case, the cursor wrap the screen. In Wayland, user can still drag the preview beyond the window, but not beyond the screen. When he try, this error appear in the output of okular: qt.qpa.wayland: Setting cursor position is not possible on wayland So, I assume the issue does not belong okular but I don't know exactly to who I should report it.
I'm on openSuse on plasma/wayland and experience the same problem. I hope this is fixable, as this feature is awesome!