Reuse does NOT always give back the 1st type of mat in the list (as said by others)

I just crafted a Bow of the Round, HQed and got a Reuse proc.

1st item on the list of mats is High Mythrite Ingot.

see my log:
You synthesize a Bow of the Round .
You were able to salvage some ingredients.
1 loop of luminous fiber

Happened again with other "of the Round" items, so far it was always Luminous Fiber I got back.

Or a Mighty Thunderstone, 1st item in the recipe ist is 2 Wolfram Ingots, the Large Levin Orb is slot 3, yet...

You synthesize a mighty Thunderstone .
You were able to salvage some ingredients.
1 large Levin orb

(Had the same happen earlier with Levin Orb being in slot 4 of Mighty Thunderstroke).

So it seems Reuse aims for a specific type of (rare) mat which is always the same for the given recipe, but NOT necessarily the mat in the 1st recipe slot. Yay getting Large Levin Orbs back :D

ETA:

You synthesize a Honor of the Vortex .
You were able to salvage some ingredients.
1 demimateria of the Vortex

Demimateria of the Vortex for this glam weapon is in the 1st slot (3 needed), the Vortex Feather would be in slot 3, the 10 Battlecraft Demimateria III required are in slot 2).

Can confirm. Been crafting a bunch of orchestrion rolls lately, and despite the faded roll being the third item on the list, that's what I get back when reuse procs.

It returns the item that (if HQ) would contribute the most to quality. If all items are equal it will return the first. This is why it will return the Nidhogg Scale on Seeing Horde weapons despite it not being the first item.