Problem with menu names when customizing themes

I’ve found a strange situation in some sites where Multisite Toolbar Additions plugin is installed: when I customize themes, via Apperance > Customize, this is what I get (see the screenshot; the plugin has been translated to spanish):

https://multiblog.educacion.navarra.es/files/2014/04/problem_menus.jpg

It seems as the HTML code added by Multistie Toolbar Additions plugin has not been correctly parsed. It happens with TwentyFour theme, and also with Genesis, Graphene, Stargazer and other themes I’ve tested.

Any idea?

https://www.ads-software.com/plugins/multisite-toolbar-additions/