Custom Categories are not saved

The ztix import just delivers so called standard categories officially used among theatres. If a theatre wants to use a more individual categorie the ztix feed stays empty:

<rubrics>
<![CDATA[ ]]>
</rubrics>

This example shows what doesn't work:

https://www.mainzer-kammerspiele.de/node/2144/edit

In above screenshot you see the custom tag "No strings attached" marked. If saved you see this tag appear as a custom rubric here:

I assume if the next import is sceduled this information is gone again. We need this information there permanently