20 stacks mejai displays as 2, drops the total ap as well

So I just had a game where I reached 20 stacks on mejai and instead of giving me 20 it fell from 19 stacks to 2. MY total AP dropped as well, I was at 600 ap earlier then it dropped to 500ish. (I did not die, I went legendary and still didn't die till game ended) Then I started gaining stacks from "2" and eventually reached another 20 and this time it showed a proper 20stacks with the right amount of ap.

Idk if it was only a display error or the actual ap given was affected as well. Match history : http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1811342987/33787752?tab=overview

As you could see I had a 19 kill streak with only 1 death. I'm sorry I can't provide a replay but please look into this regardless, since if this happens in ranked and someone goes legendary but their mejai stacks drop to 2, it's gonna suck big time :(.