What scaling factor is your display set to as this isn't something thats showing up here.
My dual monitor setup is 2560x1440 @ 125% scaling to the left (the primary monitor, where the pinned apps go), 1080x1920 @ 100% scaling to the right. My graphics hardware is an RTX 3060 Ti with Nvidia driver version 516.94.
There's no difference if I reduce the scaling to 100% for both monitors, and I still get the black square in Windows Sandbox at 100% scaling.
But I just tried Start11 on my laptop, and it didn't have this issue. That system has a single 1920x1080 display at 125% scaling. The desktop has additional jump lists for VS Code and Windows Subsystem for Linux, if that possibly makes a difference: