I was looking over the code, and spotted that the recycle recipe I presume is meant to return the original panel that was converted to a flat panel. Given what I looked at, it looks as if the ingredient is the flat panel, and the result is also the flat panel, because it uses item_name in both places, rather than base.item_name for the result of the recycle recipe.

Also suggesting to add an option to return some of the added materials used to make the flat panel. (doesn't have to be all of them, but be nice if a setting allowed for all, some, or none.)

Whoops. I actually got distracted and haven't tested that feature. I'll check that out. The feature is supposed to return all of the materials used to make the flat panel.
I added it because you can't otherwise upgrade from flat to flat, making iterative upgrading feel annoying. At least this way, when upgrading from Flat Solar to Advanced Flat Solar (for example), you can recycle the flat solar back to solar panels, then upgrade them to advanced, then to advanced flat.
Edit: Looks like I at least had the wherewithall to disable the recycle entirely.

This is my first mod, that's not just editing someone else's, so if you notice anything else in there that I could have/should have done differently, I'm open to any feedback.

Just uploaded a new version that wraps up the recycle recipe.