The menu item title form maxlength reflects the field maxlength setting

Node content entities are integrated with the Menu UI module using a form alter menu_ui_form_node_form_alter(). The Menu UI module has a special helper to get default values for the form items it adds to the form menu_ui_get_menu_link_defaults(). Previously this implementation limited the menu item titles to a maxlength of 128. This has been changed to use the maxlength set in the field definition of the Menu Link content entity.

Other content entities that implement an integration with the Menu UI module and copied the Node implementation should be changed accordingly.