When $wgAllowMicrodataAttributes is set to true, <meta /> tags are allowed in wikitext. However, Visual Editor doesn't support them (they're rendered as plain text in Visual Editor).
We use meta tags to add structured content through templates, and it renders badly with Visual Editor.
See for example this page: https://www.wikidex.net/wiki/P19
Trying to edit it with Visual Editor make <meta /> tags visible (apparently it doesn't cause any harm when saving)