Yes I have seen these horizontal chroma distortions before.
For your case with the drooping U channel, probably masktools LutXY with the options of U channel processing only. Examine the V channel more carefully it also probably has a slight droop as well.
A formula something along the lines of (((X-128) * (Y + 500)) / 600) + 128 == > "X 128 - Y 500 + * 600 / 128 +"
Choosing the values of 500 and 600 allows fine control of the U gain by the mask clips U channel, ranging from (500+0)/600=0.8 to (500+255)/600=1.25. You should choose values suited to the gain range you need to apply.
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