Big bug: why does iCloud Keychain remember the wrong expiration month for all of my credit cards?

I’ve been adding credit cards to my iCloud Keychain for years, and after a few months, I notice that my payments receive errors when I use the autofill from iCloud.

The bug:

I’ve realized that iCloud Keychain consistently changes the expiration month (and nothing else) to one month earlier.

What I’ve tried:

I used to think that I’m entering it incorrectly the first time around. But then I went and purged all of them to be correct, but, alas, after some time, the months are incorrect again.

I’ve also tried deleting these cards and adding them fresh. To no avail.


(1) Should I be worried that someone has access to my account? Or is this just a (major) bug in the iCloud Keychain?

(2) How do I fix this for good? I’ve just gone and corrected all of the months again.