On my WP Page I created a new Post Grid, where strangely no Featured Images where shown. After this I tried editing the Post Query Orderby on another grid, which worked up to this point. But after updating that grid, its Featured Images also disappeared. Why does this happen?
Btw: Due to this input field being hidden by default, it's very hard to explain ones question.
Welcome to PickPlugins.
I'm assuming you're trying to create a shortcode-based post grid, not a block-based one.
Anyway, Under Layouts, please make sure you've selected a layout selected.
Also, try to import a fresh layout from the layouts library and test it by applying it on a grid.