Rebuilt the shifter on the handlebars after soaking it in WD-40 didnt work, but the FD still snaps back when shifting down.

I’m not exactly sure what you are trying to explain. The derailleur does what it’s supposed to when you change gears it’s on a spring that pulls it back down when the shifter releases cable tension & that’s what it is doing.

If that doesn’t answer your question can you describe what you want to achieve