Now that iOS allows you to share only some photos with apps (which is great) I see this before practically every photo selection dialog in third-party apps. My choice here doesn't affect my ability to share a single image with the app in that moment; they just want to suck my whole library of photos in for background processing. I'd very much like to not see this dialog more than once a month.
Go to Settings, scroll down to find the app, and select it. Then you get the "Allow <app> to access" and a list of entitlements, one of which is "Photos". Select "Photos" and you get three options "Selected Photos" (which is what you likely have), "All Photos", and "None". Push "None" and you should never see that dialog again.
The dialog I am constantly seeing is this one, which seems to come from the OS: https://i.imgur.com/7JGotq3.png
Now that iOS allows you to share only some photos with apps (which is great) I see this before practically every photo selection dialog in third-party apps. My choice here doesn't affect my ability to share a single image with the app in that moment; they just want to suck my whole library of photos in for background processing. I'd very much like to not see this dialog more than once a month.