Thread:Support/Percentage bar always wrong in subgroup preview

Firstly sorry if this should have been filed elsewhere but it wasn't clear if translatewiki.net interface bugs should go here or into bugzilla (and under which category if so).

When browsing subgroups in the subgroup pop-up at Special:Translate the percentage bar seems to always show 100% translated, 0% proofread independent on the actual status of that subgroup.

It works correctly at the MediaWiki level but starts giving nonsense from the Extensions level onwards.

Is this bug known since before? If so is there a bugzilla (or local equivalent) tracking it?