In the recent Overwatch patch 1.8, D.Va got a buff to her Defense Matrix while Winston has a smaller head hitbox volume. But both of these changes came with some consequences, in the form of bugs and glitches.
D.Va's Defense Matrix is supposed to absord any incoming damage no matter how close the distance is when activated but the bug made her take in damage instead. Her Defense Matrix is unable to block off hitscan shots from heroes like Bastion and Soldier:76 if they are fired from behind another ally hero. Projectile shots from heroes like Mercy and Hanzo will also be able to pass through the Defense Matrix if the shots were fired under the right conditions. (Read: Overwatch D.Va's Defense Matrix bug allows her to take damage instead of absorbing them)
As for Winston, his head hitbox has a weird bug where it will shift to his elbow whenever he crouches. The bug is very likely to have originated from his smaller head hitbox volume, where it's decreased by 15%.
According to Overwatch Game Director Jeff Kaplan on his post in the Overwatch Forum, all of these bugs will only live for just a little longer as the Overwatch team has already fixed D.Va's Defense Matrix for it to work as intended, while they are currently working on Winston's head hitbox, I mean elbow hitbox. "D.Va is fixed. The fix goes live later this week or early next week. Winston fix is in progress," Kaplan posted as a reply to another Battle.net user.
There is no ETA on when Winston's fix will abe completed, but D.Va's fix is scheduled to hit live servers in a few days. In the meantime, try to avoid picking these two heroes as they can be a hindrance that stops you from your path to victory.