I honestly don't see anything unexpected here - with one proviso: You do need to view this at 100% zoom. Apparently you don't (that part of the screenshot is cut off) - and that means you get a screen artifact where line art apparently gets lighter or darker with a tonal adjustment like a curve. It really doesn't. View at 100% and you'll see.
Is that what you mean?
For performance reasons, adjustment previews are based on the on-screen zoom ratio. At less than 100%, you get a scaled and softened version, and this introduces intermediate gray values where the original is in fact full white or full black. A curve adjustment has no effect on full black and full white. They are both entirely unaffected - but on screen they appear to be affected because of the softening.
So go to 100% and try again. Then come back if you're still not happy.
This looks Like normal behavior.
In your unclipped examples the curves are darkening or lightening the entire image, including the background layer.
In your clipped examples the curves are only acting on the layer immediately below. Either darkening or lightning the pixel content. The reds go darker but still remain color. The background "white" is untouched by this curves move.
As for the histograms. They are relating the information that is available. In the unclipped example a lot of the grey/white/light tones are available from the background layer.
In the clipped example there is no solid grey/white/light tone available so the spikes on the left and right are a direct selection of what is in the layer below.
thank you very much, it is like you said. I tried the same thing on a retina monitor on 100% resolution and the artifact wasn't showing up anymore.