Sona's Passive Procs Pantheons Passive, but Still Deals Damage

The title contains pretty much everything about the issue. Sona's autoattack will be blocked, but the passive damage still goes through and hurts Pantheon.

This was the first time I've ever laned as Pantheon against a Sona in top lane, but it happened enough that I'm sure this was the issue. Panth's Aegis didn't stop her passive's damage, but it still used up the Aegis.

Sadly, I do not have video evidence or anything like that, hopefully it's a simple enough test. (She was using DJ Sona skin, if that matters.)