Derek, this has been happening at least since .913. I have this situation in pretty much every single saved game of mine, and I have a series of saves (in .98) which show Magnar going from 2 stacked blades on turn 11, to 3 on turn 22, to 5 on turn 30. I am sure that he kept going, I have saves every 3-5 turns until 90ish.
And no, Magnar did not have 5 champions on turn 30. I think it is enough that he has one in his stack, and keeps missing his target.
People have also been reporting the AI stacking Evade. This issue is widespread.