If you really, really need this fixed it might be viable to maybe implement some sort of alternative shadow casting(though note that godot is forward rendered so your options may be limited), but I wouldn't be expecting or waiting for a fix for this as is, simply because I wouldn't want to depend on a undefined potential fix date that might or might not come.
As @Calinou said tho, you'd have to deep dive into the c++ source code for this. If you can I'd instead look into using baked lightmaps. That is probably your best option and possibly benefit you also in performance.
Note that even if you need some kind of time-of-day change baked lightmaps can still work, you'd just need multiple lightmaps for the different ToD's.
