I just don't understand why I have to lock the luminance composite at 15 to prevent this. I'm not editing the highlites.
Again: There is nothing like an RGB luminance composite. The curve is individually applied on the R channel, on the G channel and on the B channel. Your bright green is saturated enough so that the low B value falls under the part of the curve which you're editing.
Instead of trying to fix this by an additional anchor point
(which may work by coincidence or not),
you might wish to consider Luminosity blend mode
- so that the curve is effecting the L channel only
(referring to the RGB > HSL separation which PS silently builds behind the scene).
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