Having said that, a bit later it stopped working, @girish - could it be, that with maybe regularly rebuilding the spam rules, the custom ones get forgotten? I still have the rule set up, checked its syntax several times (also with help of KI ;)) ... but it's not adding that custom rule right now.
(I can still verify it was working yesterday afternoon, as the type of spam had the correct rule added to X-SPAM-STATUS - while later not anymore.)