I think this is modules/ve-cite/ve.dm.MWReferenceNode.js in the Cite extension, see the "Generate a name starting with ':' to distinguish it from normal names" comment

In T92432#1862609, @Krinkle wrote:

I ran into this earlier today. name=":0" is too generic and easily causes problems when bringing in content from another page (both in wikitext and through VE) because it is basically guaranteed to conflict if both pages had at least one VE-generated citation.