Same problem reproducted.. i think it will be ok if you wait for the next miui stable (probably the 10.4), if you're in a hurry use the weekly roms witch are ok with that: tested successfully on 9.6.5 (10.4)
Its about the choice above missing in the stable 10.3 release (displayed with compatible themes only).
To test with compatible theme, search 5x6 in the theme store.
Unfortunately, i don't have the problem anymore, i tried to correct the thing by replacing the Xiaomi.eu stable (10.3) by the last weekly 10.4, problem solved like this that's why my first post came to help ..
i used 10.5 weekly now, it will be better that the one which open the post put a screenshot of the Mather.
In my memories, there were not any grid choice on the "home settings / Plus" windows.