I know this is not actually a bug, but it's still a really annoying oversight, as it makes it really hard to have wolf-assisted combat without killing all your wolves.
Basically, when you use the sweep attack it deals a bit of damage to whatever the sweep attack hits, however generally tamed wolves will be in-front of you when your fighting zombies and such, which means that if you attack a single zombie with your sword, the sword will almost definitely hit any tame wolves you have, which over time could lead to you pet's death.

This happend to me in my 1.9 survival world, I had just tamed my first two wolves that I had ever gotten in the world, but then night fell, so I had to survive walking a few hundred blocks though a forest at night, however I accidentally hit one of my wolves with an unenchanted iron sword, and then the next time I attacked a zombie the sword did a sweep attack and the wolf died

Even though it's not a bug, I think it's still something that should be fixed, (by making pets not take damage with sweep attacks, would be a good solution), because it makes wolf-assisted combat something you don't want to do if you have a limited supply of wolves, and making a type of combat discouraging is not what the combat update should be about.