When making a new recipe with CraftTweaker, using /reload to see it, and then placing the input items into a FastWorkbench to see if it works, the output will not be rendered.

The recipe does however work correctly when the empty output area is clicked.

Yep.

Is the output shown correctly if that recipe is loaded during startup?

Does that persist on a vanilla workbench?
I don't think FB does anything that would clash with a reload unless something wasn't being synced properly from the reload.

Hard to tell, doesn't FastWorkbench completely remove the vanilla workbench? ^^'
I'll have a look tomorrow :)

I cannot reproduce it in a vanilla Crafting Table. But I'll just close the issue, it's really minor, I mostly wanted to let you know in case it was an easy fix.