Include custom fields from all included projects

When retrieving custom fields to show in the filter form, evaluate all
projects included in the filter scope.
Previously, evaluation of subprojects only happened when filtering from
ALL_PROJECTS.

Additionally, check access level for the defined view threshold, to only
show those custom fields that are viewable by the user

Fixes: #5713, #23232

core/filter_form_api.php

@@ -2631,11 +2631,14 @@ function filter_form_draw_inputs( $p_filter, $p_for_screen = true, $p_static = f
 	}
 
 	if( ON == config_get( 'filter_by_custom_fields' ) ) {
-		$t_custom_fields = custom_field_get_linked_ids( $t_project_id );
+		$t_filter_included_projects = filter_get_included_projects( $t_filter );
+		$t_custom_fields = custom_field_get_linked_ids( $t_filter_included_projects );
 		$t_accessible_custom_fields = array();
 		foreach( $t_custom_fields as $t_cfid ) {
 			$t_cfdef = custom_field_get_definition( $t_cfid );
-			if( $t_cfdef['access_level_r'] <= current_user_get_access_level() && $t_cfdef['filter_by'] ) {
+			$t_projects_to_check = array_intersect( $t_filter_included_projects, custom_field_get_project_ids( $t_cfid ) );
+			if( $t_cfdef['filter_by']
+				&& access_has_any_project_level( (int)$t_cfdef['access_level_r'], $t_projects_to_check ) ) {
 				$t_accessible_custom_fields[] = $t_cfdef;
 			}
 		}