My iPhone X was doing the same exact thing. Portrait mode on the rear camera was making some severely dull photos. And if I would tilt the camera a little away from my subject, the colors came back like magic.
I also had that screen wash out problem on my iPhone X. Downgrading to 11.4.1 fixed both issues for me. Shame.
Don't mean to hijack the OP, but here's a video of the "Portrait" mode color washout clusteroo:
- "Photo" mode, on/off axis, colors look correct
- "Portrait" mode, on/off axis, colors turn to s