Index: branches/5.0/src/wp-admin/includes/template.php
===================================================================
--- branches/5.0/src/wp-admin/includes/template.php	(revision 43826)
+++ branches/5.0/src/wp-admin/includes/template.php	(working copy)
@@ -1096,6 +1096,8 @@
 					if ( WP_DEBUG && ! $screen->is_block_editor() && ! isset( $_GET['meta-box-loader'] ) ) {
 						if ( is_array( $box['callback'] ) ) {
 							$reflection = new ReflectionMethod( $box['callback'][0], $box['callback'][1] );
+						} elseif ( false !== strpos( $box['callback'], '::' ) ) {
+							$reflection = new ReflectionMethod( $box['callback'] );
 						} else {
 							$reflection = new ReflectionFunction( $box['callback'] );
 						}