At https://commons.wikimedia.org/wiki/Category:South_Pole_Telescope there is currently a link to 'Wikimedia Commons' in the sidebar, under 'In other projects'. That's not much use, given that the user is already on Commons!
Removing the value of P373 from the linked Wikidata item ( https://www.wikidata.org/w/index.php?title=Q1513315&diff=938513067&oldid=919250761 ) removes the link, so this is to do with P373 rather than the sitelink.
The same behaviour was also present at https://commons.wikimedia.org/wiki/Category:Albert_Einstein - which was resolved by this edit: https://www.wikidata.org/w/index.php?title=Q7213562&diff=938511809&oldid=938511640
If a quick fix is possible for this, that would be good. A general fix would be to stop using P373 in the sidebar now, given that most Commons categories are now sitelinked either directly in the topic item, or indirectly through the corresponding category item.