Yes unfortunately it is. Getting WPF windows to be fully chromed as we do requires a pretty complex configuration of Win32 settings, Win32 message handling, and WPF logic. In this scenario, we are not sure why Windows 10 seems to be retaining a small strip of the WPF visual that was there on the other monitor before maximizing on the monitor the window is mostly over. When testing this in the past, we did find we could remove that issue if we used clip regions. However here's where the complexity comes into play. Every Windows version seems to change things and we found that if using a clip region when maximized in a certain scenario, Windows will render "classic" title bar buttons on top of our real title bar buttons. Also using a clip region can trigger some overscroll rendering issues with WindowsFormsHost. It's unfortunately a very delicate balance and the best choice given the options we had were to live with the small rendering strip when maximizing a window that is split over multiple monitors.
All that being said, the good news is that Microsoft changed something in Windows 11 and the problem does not occur there.