The title says it all. If you install the FastWorkbench mod and equip a backpack, then when crafting something in the player's inventory, the result of crafting will not be displayed, but the "crafted" item can be picked up.
For example, let's take an oak log and place it in the crafting grid. In crafting result slot, oak boards are not displayed, but they can be taken by clicking LMB: https://ibb.co/fHJNj33

Hello! Quark's moved to 1.18 now-- unfortunately, this means that (unless your bug is critical and/or game-breaking), we're closing support for issues filed for versions prior to it (e.g. Quark on 1.16).

If your issue still occurs on 1.18, or if your issue happens to be rather critical (e.g. consistently crashing the game with no other mods present), please leave another note or open a new issue. Thanks for your cooperation!