Not sure if it's well documented (not many shield tanks out there, these days) and it's not in the Known Issues list; the Fighter's Stalwart Defender stance +HP% bonus does not stack with the +20% HP bonus in Unyielding Sentinel.
Tenacious Defense: Stalwart Defender tier 3, 4, or 5 enhancement
- While in Stalwart Defense, you gain a +10%/+15%/+20% Competence bonus to maximum hit points.
Strength of Vitality: Unyielding Sentinel tier 5 passive ability
- Passive Bonus: +10/15/20% Insight bonus to maximum hit points.
I tried getting both of these on my fighter/paladin tank, and they don't stack. I tried resetting enhancements/destinies and reapplying it, unequipping all items (except shield, which is required for fighter's stance HP bonus), and all sorts of checks to make sure they didn't stack. It's probably one of two things: either the fighter's HP bonus is insightful or the sentinel's HP bonus is competence. The paladin's sacred defender HP bonus from their stance is a sacred bonus, and that's been confirmed to stack with the sentinel bonus.
I made a thread in the general chat section about this back when Update 21.1 first came out, and other fighters confirmed that it doesn't stack.