It is a widespread bug in the latest release of Google Photos App. It affects Xiaomi devices, some Huawei phones, Razer, etc.
They will most likely fix the issue with an update.
Until then, there are a few things you can do:
1. Go to settings, display, contrast and color, then change from AUTOMATIC to STANDARD. This will fix the issue, but it means you can no longer use the phone with automatic contrast;
2. Uninstall Google Photos and search for an older apk online to install. This will solve the issue as well, but you will lose the most current features of Google Photos. For some reason, I had to install 4.14, I believe, because the latest (4.21) had the contrast and darkness bug you are referring to, while 4.20 to 4.15 were always crashing on my device.
These are all compromises until a new version of Google Photos is released.
