Here is proof that during 1.7 (long before 1.12) there was fix to remove all spellpower scaling from self healing items:
All item based self heals will no longer be effected by equipment that provides bonuses to healing. This change affects both the Arcanite Champion and the Crusader Enchantment.
The diamond flask was missed because it was added in 1.8. Clearly this was an oversight.
edit: per the most recent patch notes, Blizzard has stated the following:
Developers’ notes: As always, our intention is to accurately represent the 1.12 design intent of the game, and these bugs are clear examples of behaviors that were never intended.
Snapshotting, 100% +healing scaling on a warrior quest trinket is quite obviously not an intended mechanic.