As it stands right now, creatures with defender that take lethal damage on the same turn that the player takes lethal damage don’t die. This is confusing; I get that creatures don’t die to combat damage until all damage has been assigned, but why should a planeswalker that takes lethal damage after a creature die before that creature? That just seems wrong. The planeswalker and the creature should both die, at the same time, at the end of combat.
Obviously this is a small issue, mostly just occasionally affecting “Kill X or more/less creatures” bonus objectives. It may not even properly be a bug. But it’s definitely unintuitive.