How to reproduce it:

1. Enable EVENTS.EVENTS.NEW.UPLOAD_ASSET.OPTION.SUBTITLES={"id": "track_subtitles_option", "type":"track", "flavorType": "captions","flavorSubType": "source", "tags": "generator:unknown", "displayOrder":3, "accept": ".vtt"} in (/etc/opencast/lisproviders/event.upload.asset.options.properties)
2. New Admin UI
3. Event Details --> Assets -- > Option Add Asset is missing.

In the old Admin UI looks like this:

Opencast Version: 15.5