When using Sidecar on MacBook Pro, built-in display is tinted I don't know why, and if this is expected, but I noticed that the built-in display on my MacBook Pro was tinted, as if the system was applying another and incorrect color profile on it, when using Sidecar.
I had the exact same problem. Disabling True Tone should fix this.

I believe this is a graphic bug at the moment.