Impressive find @azaozz !
I hid the author box on the Edit page and seemed to restore functionality. I’m not sure which plugin broke it…but disabling all plugins / switch theme did not resolve the issue. Adding this to the functions.php resolved it:
function remove_default_post_screen_metaboxes() {
remove_meta_box( 'authordiv','post','normal' );
}
add_action('admin_menu','remove_default_post_screen_metaboxes');