I have noticed the same thing. It does not just happen in bright ambient light, but also at lower brightness levels. However, with bright ambient light the effect becomes stronger. I haven’t really noticed it in photos, but it is really noticeable on small text, especially if this text is neither pure black nor pure white but some greyish colour. To me it looks like overshooting caused by a sharpness filter.
I did some experimentation and the effect cannot really be captured on a screenshot. If I display the screenshot at 100% on the ipad screen itself, I can still see the effect. But as soon as I zoom in or view the screenshot on another device, the effect is not visible. This indicates that it’s some sort of postprocessing that only happens right before the image is fed to the screen.
I tried to capture the effect by taking a photo of the physical screen using a camera (screen brightness was only medium):
The effect is particularly noticeable in the small caption “brightness” as well as in the help text “Automatically adapt iPad…”.
I also have an iPad Air with an LCD screen here for comparison, there the effect is not visible at all, and the colours look even. I find this pretty disappointing, especially since this screen was advertised to deliver a very high fidelity. I thought about contacting Apple support about this, but I have not done this so far. If the assumption is correct that this is caused by some sort of image postprocessing then it could hopefully be fixed in a software update.
Do you know what would be the right address for reporting a problem like this to Apple?