Featured tag can not be set in editor

This is a rephrased description of
https://www.ads-software.com/support/topic/tags-disappear-in-editor-but-not-in-quick-edit/
I think it might be a Twentyfourteen problem, since the filters being used are defined in the 2014 code …

I am using the default tag named “featured” for featured posts.

Now when editing Posts, I can not assign this tag “featured” any more in the the tags metabox in the right column of the editor. This has been working up until just recently. If I add the tag and hit enter within the tags field, the tag is there just like the others (grey box with an X). As soon as I save the post, the tag disappears. Also, when I type the tag name, the autocomplete fails, giving me no indication that a tag “featured” exists.

If I try to assing the featured tas using the QuickEdit in the post overview, everything works as expected.

then I remembered that there is a setting in the Twentyfourteen Customizer ( in the “Featured Content” section ) which is supposed to hide the special featured tag from being displayed in the frontend. If I disable the setting, the feautured tag can be assigned in the editor.

So as it stands this customizer setting does not only hide the tag in the frontend but also prohibits the tag being set in the editor.

Regards
Uli