Are you sure the other classes are gaining barrier through the runes at full hp and not just through skills or traits that grant barrier? There's a lot of barrier around these days, and available on other runes too. (Scrapper, Holo, Weaver, Scourge) Mount - Pounce. Runes - Rebirth, Scourge, Weaver. Of course Sanctuary won't grant barrier at 100% hp to you, you're not doing any healing.