Quick Synthesis should look at your enabled skills. If you have Careful Synthesis (or Careful Synthesis 2) either because you are that class or because you have slotted it in as a cross class ability, then it should be using that rather than Basic Synthesis.

That said - Basic Synthesis has a 90% chance of success. That means, a 10% chance of failure. If the 40 Durability recipe takes one Basic Synthesis to complete, then Quick Synthesis should currently have four shots at finishing it successfully - and though not impossible, the odds of failing all four are 10% x 10% x 10% x 10% ... or 0.01%. We should not be failing more than one in one thousand if the Quick Synthesis is using Basic Synthesis and it only takes one to finish it.