I came across this bug today when i was raiding. The opponent had Aegir so i used Wilbur to counter it. The fight went on, and near the end i realised that my heroes share the dmg but dont have the defense buff. So i fired Wilbur again and took this pic right after. Its clearly visible that there is no defense buff active, only the spirit link while the defense debuff on the enemy Sartana is working as intended.
Update: In my next raid i also used Wilbur and when using him for the first time, the defense buff worked but the enemy did not recieve the debuff.