The same affix value on the same unique, on two different items, has two different outcomes

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What went wrong?

Detonating Arrow costs 8 mana. I have 2 Jelkhor’s Blast Knives that have “-4 Mana Cost for Detonating Arrow”. Upon equipping both, Detonating Arrow says it costs 1 mana. Upon further investigation, equipping and unequipping the daggers reduces the mana cost by 4 and by 3 alternatingly. For instance: equipping the Jelkhor’s with 2 legendary potential, the tooltip shows a mana reduction by 3. If I equip the Jelkhor’s with 0 legendary potential afterward, the tooltip shows it costing 5 mana (still reduced by 3). If I unequip and reequip that Jelkhor’s with 0 legendary potentital, the tooltip shows it costing 4 mana (now reduced by 4). If I unequip the Jelkhor’s with 0 legendary potential and equip the Jelkhor’s with 2 legendary potential, the tooltip says Detonating Arrow costs 4 mana. If I uneqeuip and reequip the Jelkhor’s with 2 legendary potential, the tooltip then shows it costing 5 mana.

[Log File]
Can be provided if necessary.

Screenshots (via a site like imgur) or video, if applicable
Here are several videos showing the change in tooltip when equipping the particular items. https://imgur.com/a/aKqmo19
And the other videos show the corresponding mana cost in relation to the tooltip.

Addendum: Unfortunately I can’t manipulate a state of -4/-4 when trying to dual wield (which is what I was originally trying to do). And dual wielding always comes out to -7 :\

More2: The original daggers I did this test with were made through using Rune of Ascendance(s). I ended up farming another dagger, interestingly with the stats of -4 mana reduction for DA and 2 legendary potential, and it worked as expected :))))

Thanks!