I wouldn't say it's AST's 'mechanic'. It's purely how their ability "Essential Dignity" behaves, which is:
Restores target's HP
Cure Potency: 400
Potency increases up to 1,100 as the target's HP decreases
That being said... what you're seeing is probably a healer who tried to be efficient with their healing
(or simply being bad, who knows). While this is mostly seen as a sign of decent healer
(or a healer trying to improve themselves), by no means it's not all sparkle and rainbows. Get overenthusiastic, they might end up misjudging & killing you in the process.
(Which is what happened most likely in your story)
OR they simply didn't use their Essential Dignity for whatever reason, because it rings suspicious to me when you said it heals from 10% to 30% in Brayflox. Essential Dignity is absurdly powerful heal under lv50 that it can heal tanks back to full (or really close) especially when used on that low HP%. Up to 30% looks more like a Benefic I or Benefic II with some broken gears...
I don't think you should stop rolling as a tank in DFs though, but perhaps a nice break or just kidnap one of your healer friends
