in our case we do not have “classic editor” or any other plugin like that installed. the site uses block editor and elementor.
but i have another addition: even when trying to generate a new field group with a “post object” and using the post type filter (which uses select2 as well) the problem occurs.
so it is not only in the posts and options pages, but also in the acf admin area.
thank you again for your help 🙂