I've had a couple of bugs that appeared after updating to 11.4.1. Most notably, I am no longer getting push notifications to my iCloud mail account via the native Mail app (am still getting them through Airmail, though). I signed out and into my iCloud account again as per the Apple Support instructions, but I think that did more harm than good to my settings. Now, I'm constantly being asked for my password to view my App Store account on my iPhone, and it won't work. My iPad account works fine, though.
EDIT: The email issue still persists, but I figured out the login issue - I had to log out and into my account another way. I swear, I feel like there are too many different ways we are forced to log into our iCloud accounts, some less obvious than others.