Healing with some belts is much less than expected

I am experiencing an issue with expected healing values for grenade belts.

Currently, my character's is level 35 with 2380 hit points and a passive 20% bonus healing buff from an enchantment on my chest. I am not using any other equipment that affects health/healing/etc (such as The Burden).

I get similar results to the image on the left when using completely different equipment, no enchantments, unupgraded starting gear, different companions, and no skill points assigned. I am playing a Mage.

This issue isn't just isolated to the Warden's Straps item, other belts give the same result, only a few belts such as the Tincture Kit give values in line with their expected value, the issue seems to be isolated to the grenade belts.

With a 20% healing buff on top of the 815 healing from the Warden's Straps +8, I fill 2 bars from empty. Each bar seems to be worth 225, so I am only healing for around 450ish, instead of the 975ish that would be expected.

I get an identical results with my Legendary Crow's Bandolier +8, which has an identical healing value, as well as with the other grenade belts that have lower healing values due to being less than +8.

The only belt I get less healing with is the Worn Sash starting belt, which heals correctly for the listed value of 400.

Loading saves from much earlier in the game, I found that even +2 grenade belts restored the exact same amount of health as my later game +8.

I asked another player to try and replicate the results and they got the expected healing values from these items at the same rarity, upgrade level, and build.

Based on my results, I believe that the grenade belts are only visually +8, and in actuality they're all giving the healing values of their +0 variants.