When cataloging if you have CCODE as required in your framework Koha will automatically populate that field with the first option in the CCODE pull down. This seems silly to me and kind of defeats making it required. You make it required so that you're prompted to fill it in if you forget, but if it's filled in already then you might forget to change it to something else. I think that if this field is required it should still default to blank (like it does if it's not required) and prompt you fill it in if you try to save without selecting an option. Nicole PS. This is 3.6, 3.10 and master (probably even before that as well)
I can reproduce this bug in 3.10.x but not in master as of 3.17.00.009