Steps to replicate the issue:

• Select Vector skin (e.g. open https://en.wikipedia.org/wiki/Special:BlankPage?useskin=vector)
• Add a new portlet following the first example from T303488: [Gadget support] Add a Javascript function to create a new portlet section similar to mw.util.addPortletLink:

var x = mw.util.addPortlet('foo');
$(x).insertAfter( '#p-navigation' );
mw.util.addPortletLink('foo', '#', 'New portlet link');
mw.util.addPortletLink('foo', '#', 'New portlet link 2');

What happens?:
The style of the new links is different from the existing ones:

What should have happened instead?:
New and existing links should have the same style: