Windows Version: Win 10, 2004, 19041.1320
Pressing 'Check for Updates' on the about page reveals that my copy of Fences is up-to-date
Open a few windows (any with a textbox on it such as chrome's or windows explorer's search bar) and then press Win+D to hide all active windows and then select a window from the taskbar, select the textbox to type text and press 'D'. You may have to repeat these steps a few times to reproduce the issue but it seemed to happen a lot to me until I turned the feature off in the about panel.
I usually pressed that combo to hide all open windows and then I would select only the one window I needed open. Maybe it has to do with some variable in fences 4 that isn't being set or something. Strange behaviour but its my best guess.
If I get more time I'll install a screen recorder and upload a video if you're still not able to reproduce the issue.