Building tank, enemy team only needs to do half my health to kill me.

Been playing league for about 5 years now, in the last year or so I've come across an issue where I get this bug that cuts my max health in half when building full tank. Most recently happened Aug. 23rd 2016 in a game as Warwick in aram. Once I get around 2500+ plus health, enemies only have to do half of it to kill me, it doesn't matter how much more armor, mr, or health I build, they only have to do half of my health. I've experienced it in multiple games while playing as Garen, Malph, Leona, Jarvan IV, Renekton, and probably more. I noticed happens at least 90% of the time with Renekton if I build full tank (including mastery page). which is when I first realized it about a year ago.