So far Barrier/Force Field spell worked like 100% damage reduction against everything except Spirit. Unless and until there's a definite evidence that it works differently in DAI, I'll consider it working the same
Wrong.
You must be either very confused or naysaying (as usual, apparently) based on well, not much. Why am I not surprised? Allow me to educate you because isn't learning fun?
You gain an extra layer (blue outline) which takes all the punishment while your health (green bar) underneath stays intact. It's not "damage reduction" when your health isn't affected.
Damage reduction would be: "this attack causes me 10 dmg. With Barrier active, it causes me 8, or 5, or 2, whatever". Regardless of them calling it "additional health", which would be, well, additional health points to your health bar. If you fire a swarm of bullets at me and I am unharmed, not a hair out of place, not a wound, no dmg what so ever but to said barrier, then it's technically making me "immune" to damage over a short period of time.
The blue bar depletes on its own over time and/or when it takes enough damage. But as Adhin pointed out previously, apparently immunity isn't a real word. So I can see why they're calling it "additional health", and not making you immune or invulnerable for the duration of Barrier (like similar force-field spells); but let's call it what it is. You, however, may call it what you will. It changes nothing.
And the Spirit thing: I've NO idea where you pulled that one from. O.o