If I understand the scenario correctly:
"A" has a pet that applies x% damage reduction de-buff on "B" through pet passive. "B" losses any %damage buff he gets from his pet.
In that case Azebor is right, its not a bug. This is how it works, debuffs overrides buffs irrespective of %.

Reference:
https://www.spacetimestudios.com/showthread.php?157892
https://www.spacetimestudios.com/sho...Override-Buffs