I knock a player, team mate steals kill, should give me the kill

Pretty much what others said. It works exactly like other Battle Royales - the person that downed the player is the one that gets the kill. If you didn't get it, then someone or something else had to deal the final hit - be it teammate, the zone or even his own explosives. Or the downed player got revived in the meantime or revived himself with the golden knockdown shield. To be fair, in a big gunfight, confusion regarding who got the kill can happen due to kill/knock prompt being out of line of sight (thankfully; I don't think anyone would want it to obscure vision), so I can see why it can be confusing.