Forge Weapon Duration - Shrapnel Talent calculating incorrectly?

I am trying to create a Forge Weapon build, and I would like to use the Shrapnel talent to juice its damage a bit.

Issue is, when I apply it, it seems to reduce the weapon duration too much.

I currently have 107% increased duration (total 207% duration):

  • 20 + 17% from 2x Phantom Grips
  • 10% from Folded Steel
  • 60% from Well Forged Weapons

When I do not use Shrapnel, the Weapons last approx 41 seconds as expected (timed) (20 sec * 207%)

When I apply Shrapnel, their lifespan drops to ~17 seconds. I can’t figure out why:

  • If it was simply 41 sec * 25% then it should be ~10.25 seconds duration.
  • If it’s additive with the other modifications, the duration should be 20 * (207-75) = 20 * 1.32 = 26.4 sec.

I’m not even sure what I need to change in my build, as I can’t figure out how the duration is calculated.