The header image gadget currently floats the image below the statement section if there is no space for the righthand sitelink bar. It is very out of place there. We don't have a good alternative design so for now we hide it when it would float below the statement section.
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This would do it: https://www.wikidata.org/w/index.php?title=MediaWiki:Gadget-ImageHeader.js&diff=475085472&oldid=464604137
It works for me. I'm calling it done. Feel free to re-open if it doesn't work for you.