I tried to display some dummy text instead of quiz and it is also not show. Is the content plugin mechanism correctly working on your element? Can you please check if any other standard content plugins working fine for example Joomla articles content plugin?
But anyway, I will try it myself now and let you know.
When the shortcode is not present, there is no plugin getting executed so the text is displayed. So your zoo field is working but not the functionality to trigger content plugins.
the zoo plugin might be wrongly replacing the content returned from the content plugins. Please note that this behavior is tested with joomla default plugin (loadposition) also. so it is not a problem from quiz plugin.
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