All forms of healing will produce reduced amounts with gear slots filled. Removing gear slots improves healing. See this video for demonstration. This applies to every form of healing (flasks, life on hit, warmth regen) that I could test on my character.
The healing is also significantly less than implied by the item even without all gear, either unaffected or inverted (reduced instead of increased) by willpower/healing received/barrier generated.
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Thank you for reporting and making the video. Having the same issue on my necromancer; potions, blood orbs, undying aspect, even the lucky hit proc on the pants themselves are drastically reduced when the heal goes towards barrier.
By my calculations the barrier amount = heal amount x base health / maxmimum health
so percentage based heals always heal based on base healt value, but flat number heals are actually reduced by having more max health