Went to add another photo, and overlay slid down advising to sign in.
Signed in again and added another photo
Subsequent photos were added without the need to sign in again.
So the problem appears to be something to do with forgetting the first sign in.
However - the above was retried using the private browsing mode of Firefox and it did stay signed-in. So this problem may be hard to reproduce consistently.
What occured
So the problem appears to be something to do with forgetting the first sign in.
However - the above was retried using the private browsing mode of Firefox and it did stay signed-in. So this problem may be hard to reproduce consistently.
Platform
Firefox Daylight 30.0 (3279) / iPhone 7