If I remember correctly Drifter from assists gets 40% of his stack from kills. Before it was 5% per kill and 2% per assist and now it's 4% per kill and 1.6% per assist, but showing only the whole number.
I got this bug reproduced like 4 times in 10 minutes, basically it triggers if you are leaving the healing area while sliding right at the same time as medicinal specter ends.