You mean by the edge of the spillover where the water falls toward the tile wall?
It looks like the spillover is wedge shaped with the water entrance smaller then the spillover exit. The water flow loses velocity as it expands in the wedge and so the water on the side has a different flow then in the middle. I don't see how you can fix that at this point other then increase your pump speed and water flow rate.
Your still having water adhesion to the stone. Try drying it off and then apply wax to see if it helps it gain velocity. If so polish the stone or seal it with good sealer to keep slippery
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