Just chiming in here as I have a similar issue with that exact same items.
I have 2,920 health.
I am wearing the Warden's Straps which should be healing me for a base of 1,057.
I have the Pursuant's Implements equipped which should make those 25% more effective.
I should be healing for about 1,321 per potion, which is 45% of my health bar.
In testing, when I use a potion, I heal for 21% of my overall health.
So either it's a bug, or the Healing value on a Grenade Belt is not indicative of the true number you're healing.
