Mejai's Soulstealer stacks resets to 0 bug

During the team rank with my friends, I was playing Nidalee. I bought Mejai's Soulstealer as my second item after Morellonomicon. In the middle of the game, I remembered it has at least 8 stacks. But it resets to 2 stacks after I killed one. Which means it sudden has 0 stacks. Then the bug does not appear in that game. I screenshot 2 pictures that can confirm stacks has reseted