User blog comment:MisterWoodhouse/Social Activity, the 2010 Editor, and Unpausing Migrations/@comment-34918040-20201012232815/@comment-26939652-20201013002539

Try: 0.


 * Docs: for use in mathematical situations: it's reliable and should be used only to deformat numbers which are known to be formatted exactly as formatnum formats them with the wiki's locale.

So the fix is to first normalize to formatted version and then deformat.