Grasp of the Undying and basic attack blocking

I was playing against a Pantheon in top lane, and was able to proc Grasp of the Undying even though he had his passive up and blocked the attack that healed me. However, he did not take any extra damage. I was wondering if this partial block is an intentional decision by Riot or if this is a bug. Personally, I am happy that I can still heal from it as it helped me significantly during laning phase and it did block the damage so it didn't cost Pantheon anything and still technically blocked it.