Not sure if this is a bug or a feature, but it seems a little too good to be true:
I can tinker tools (at least those with a progress bar) while working on a project, including tinkering the specific tool I am currently using on a project task.
you can do most actions while working - cooking, tinkering, giving, eating, etc. this means you can be fruit changing for a food requirement while "working" on another. This being the case I'd think it is more likely not a bug.