I have a paladin with a negative group assassination score. Equipping him with an item that yields group assassination correspondingly leads to a reduced individual assassination score. Unequpping, reload, equipping did not help.
Since this is in the ongoing ladder game 18984, I've sent more details in a PM yesterday, but I did not get a response yet and my turn time is running out, hence I post it publicly in the bug forums now, in accordance with the bug report guidelines. Thanks!