Draven occasionally doesn't get damaged by Thornmail (or takes heavily reduced damage)

I had two Thornmails on a Darius 3*. A couple opposing teams had 1-2 Dravens on them. Sometimes, but not always, Dravens would attack the darius and take seemingly no damage, or take very little damage.

At first I thought it was because a Draven had a Phantom Dancer (was Thornmail damage considered a crit that it was dodging?). Then I went up against a Draven 2* that didn't have Phantom Dancer. This one had Bloodthirster. It took about 24 damage a hit, which is significantly under the >=20% return (70% return from double Thornmail, minus up to 50% from Bloodthirster lifesteal... however Bloodthirster lifesteal is reduced due to Darius' armor and knight buff). Then I went against the Bloodthirster Draven later on and he quickly killed himself on Darius. Imperial buff was not the change there. For some reason he took the normal, expected amount of damage on some rounds and not some others.