Confusion about kindred and her marked targets

This has happened a few times to me, I mark a target, then when i kill them I dont get the stack for it. Does marking them not mean yo get the stack for killing them? Does that just make it where you get a chance of marking the champ you selected or what? I am a little confused, so if someone could dumb it down for me that'd be great.

Or is it just a bug? I have the recording saved, and i watched it a few times to make sure i wasnt wrong, but I didnt mark my target, get an assist or kill, and not get the stack for it. Please explain. Thanks in advance