I've had this issue for the last three iterations of Start11. Just installed the latest version (1.24) and the issue persists.
I have the taskbar positioned at the Top, but as you can see in this screenshot the top of the thumbnail for open app windows is cut off, making it impossible to close the open window because the X is cropped out:
I am hoping that a fix comes soon because as banal as it seems, I find myself sorely missing this functionality.
NOTE: I'm on Windows 11 Home, 21H2 (Build 22000.675)