Setup: a random grid with post type Media Library and no selected images. The help text tells us that “If no image is added then all images will be displayed.”.
When using PHP7, this will show all Media Library images in the resulting Grid, as expected.
When using PHP8, the grid will show an error message “No post was found with your current grid settings”.
The only workaround is to select some images to display. Those will then be shown in the grid. However, manually selecting images is unfeasible when working with large media library’s.