Push option merge_request.assign doesn't work

I’ve upgraded to 13.12.3-ee, and merge_request.assign was introduced in 13.10 (as per the docs). I’ve tried setting it to the target user’s gitlab username, and to their full (quoted) name, but neither works. This gitlab issue tracker issue states that it should be set to the gitlab username. How am I to use this feature? Am I missing something? Or is it just broken?

Hi @ramblingshark
last time I have tried it as part of this thread I wasn’t able to get it working either.