I have the same issue, and my friends who fly with me have also experienced it. In most cases, it's overexposure; the screen suddenly gets very bright and returns to normal after a while. But in a few cases, it's underexposure. For example, when I got into the JF17 cockpit yesterday, the screen was severely underexposed, and it took about half a minute to return to normal.
I'm using Pimax Crystal.