It could be that the tip shape is causing the differences, but...

Many of us here have done experiments with different wing tip shapes, tip plates, winglets, etc., and they usually don't make a visible performance difference in .46-size and smaller aircraft. (Do a search here in "Aerodynamics" for some of those discussions.)

It's more likely that in the process of changing your wing tips, the weight was changed, causing a CG-related issue.