Banshee's Veil - Bugged or the Tooltip needs Clarification

As a game with Swain, Two enemies bought Banshee's Veil and I thought I could get rid of it with my Q, Decrepify.

It did not proc the Banshee's Veil and I think that's stupid, cause even if intended, there was no way for me to know that because the tooltip does not tell me that.

The item does something extra that is not stated in the tooltip and that is just frustrating.

Also, why doesn't damage over time proc it? Ambient Helmet hurts damage over time champions AND Banshee's Veil does too? That's too much overlap especially if they build both!

To be fair, I don't care if it's intended, but it tilted me pretty hard that I put myself in bad positioning because of the tooltip being misleading.