Ok we're missing something. The idler cage tensions the chain so there's a force pulling it forward. We need to find something pulling it back. Unless this RD has a sprung upper body it should always go all the way forward, unless a tight pivot held it. But I've never seen one spring back as yours does (except for those with upper springs).
One other possibility but it's so obvious that I hesitate to mention it. Is it at all possible that the RD housing loop is short enough to pull on the adjuster?
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