I have what may be a fix for some of you. Beginning in March, right after there were two iCloud outages, two of five devices stopped syncing Bookmarks, Reading List, or iCloud Tabs. Everything I tried temporarily resolved the issue, but after a few hours sync would break again.
The two devices where sync broke are an iPad Pro and an iPhone 12 Pro. The devices that were syncing correctly are an iPhone 12 Pro Max, a MacBook Pro, and a Mac Studio.
Earlier this week, I backed up my iPad, put it into DFU mode, and restored it with my Mac. This wiped the iPad, including the OS, and re-imaged it. I then restored my backup. Interestingly, prior to putting it into DFU, I tried logging out of iCloud on it about five times. Each time I’d have to force restart because it hung. I think it was getting hung up on removing iCloud Keychain, which may be related to the sync issues. I gave up trying to logout. Once I restored the backup, I was able to log out of iCloud and delete the Keychain. I then signed back in.
So far, sync is still working between the iPad and the MBP, Mac Studio, and 12 Pro Max.
If things look good in about another week, I’ll DFU my 12 Pro as well. (The only reason I haven’t yet is because re-adding credit cards to Apple Pay is a pain. On more than one occasion Amex has blocked me from adding them back, which then required calls to Fraud Services.)