Is this a bug?

I was playing this game, and we were winning pretty hard, but late game I noticed that I wasn't doing much damage to Diana or Veigar, who were pure AP until the very last moment. It got to the point where I had a much easier time bursting Vlad, who was really tanky, than either of the full-on mages.... even when Soraka wasn't around, or just dead.

I know Diana has a shield, but if you look at the stats, is it normal for her to be tanking so much damage despite building full AP? It also felt like Veigar was tanking WAY more than he should. Is this a bug or am I just an absolute moron?

In hindsight I definetely made some mistakes, I rarely play after all, I'm simply concerned at how 'squishy' mages could stay alive for so long in those fights.

Keep in mind their zhonya's wasn't picked up until the last few minutes.

*TL;DR Diana and Veigar felt much tankier than they should have been.