What's Martyrdom supposed to be?

I’ve a few threads that agree that the tooltip or the buff is broken, but can’t really find what it’s supposed to be.
Currently the tooltip says, “Increases damage by 5% for every 15 health missing”.

I have seen the follow variations:
“Increases damage by 5 for every 15 health missing”
“Increases damage by 5% for every 15% health missing”
“Increases damage by 5 for every 50 health missing”.
Doesn’t work at all

To add to these I’ve also heard that’s it’s a UI bug and is in actuality 5% power per 15% missing health, the stacks just don’t display properly. I also noticed that when healing with Medicae from low health the stack amount doesn’t update for a while (no idea if it consistently does that).

I don’t think it’s something most Zealots play around anyway but it sure would be nice if it/the UI worked properly.

@Aqshy any word on this? Seems to have remained in its bugges state since pre release beta. Can we get confirmation if it’s a UI bug, how it’s intended to work, and if it’s on the radar for fixing at the moment?