If the owner of a wolf is attacked by another player, the wolf will not defend its owner as expected. When the owner's combat tag expires, the wolf will then attack. But it will never attack while the owner is combat tagged. If the owner is killed by another player, removing the owner's combat tag, the wolves will attack whoever killed their owner.

This bug isn't present for operators. If the attacker of a tamed wolf owner is an operator, the wolves will attack him as usual.

Interesting, thanks for the report.

It seems tamed animals won't attack players in the same team as their owner.
I'm not sure if this is minecraft behavior or spigot behavior.