I have experienced the same thing. Even when Apple made changes in Keychain policy to try to combat fingerprinting, “I never got the memo.” That sounds nuts, but I’m in the same boat.

I’ve had a few apps I’ve redownloaded months later, the only one from the developer, and my auth state was preserved.

I keep hearing that the Keychain data should be deleted, but my iCloud Keychain is filled with long-dead data