Kindred mark

When playing kindred in a few recent games, i noticed that if i walked up to a lane to gank and marked about 2-3 seconds before i actually attack the enemy champion, that even if i get the kill/assist i dont get the mark. this usually happens if i mark the target and get only a few attacks in before said target dies. At first i figured there was just a timer in the tooltip that states a certain amount of time before the mark actually activates and applies to the kill/assist. However, when reviewing the tooltip, there is no such timer stated. Weather this is a bug or just a hidden mechanic, it's very misleading as i just lose out on an opportunity to get a free mark on a free kill. Thank you in advance to anyone who looks into this mechanic and hopefully can deliver insight as to what is happening in these cases