Kayn Passive stacks not working?

So I was playing Kayn in jungle, and my bar to transform was almost full (looked like 1 or 2% off), so I decided to say "f*ck it" and just walk into a lane to deal damage to a champion, get my stacks and then back to base. So I deal damage to the laner, see some red dots on the floor, these reds dots fly over to me... (was at least 4)

And nothing. Didn't get any passive stacks even though the dots clearly made it to me. Took me 2 more minutes to finally get my transformation. Similar instances have happened multiple other times.

Is this a bug, or am I just not understanding his passive correctly?