If you kill a wolf and give it a bone during its death animation, you get a tamed dog that is red and sits diagonally, jittering back and forth in its death animation. It behaves like a normal dog, you can make it stand up and follow you, etc., it just looks like it's dying all the time.
After reloading the world it's just a regular dog.
Tip to reproduce this: Put a lot of wolves in a 1x1 hole, take a weapon in your main hand and bones in your offhand and spam-click both buttons. For easier results enchant the weapon heavily or give yourself a big strength effect. You can also put

/summon Wolf ~ ~1 ~ {Health:0}

on a clock and give the dying dogs bones.
Idea for a fix: Someone at Mojang could do complicated stuff with the wolf->dog conversion, but there could still be other bugs with perma-dying mobs in the future. Better just let dying mobs not have a hitbox. Maybe just completely replace the death animation with a particle animation.