From the Teamcraft Discord server:

in case anyone is interested... quick synth failing when you are above recommended craftsmanship (and appropriate level of the craft) appears to be tied to having an older offhand equipped. we did a bunch of testing and while i couldn't reproduce this, Nat was able to. 2 out of 5 lumbers broke wearing indagator gear and level 84 offhand. swapping to a lower level main hand was fine.

so if your quick synth is failing... upgrade your offhand.