I doubt it's anything to do with old versus new @ suffixes.
My money is on brute force attacks... or an internal system stuff up of some kind.
If the former, it's possible affected users use their apple ID email for other apps and websites and one of those sites/apps has been hacked.
I changed over my apple ID email to a unique email that is only used for my Apple ID several years ago because of this very situation.
And if it's not the former and is the latter, then it's still a good idea to use a unique email address for your Apple ID.
My money is on brute force attacks... or an internal system stuff up of some kind.
If the former, it's possible affected users use their apple ID email for other apps and websites and one of those sites/apps has been hacked.
I changed over my apple ID email to a unique email that is only used for my Apple ID several years ago because of this very situation.
And if it's not the former and is the latter, then it's still a good idea to use a unique email address for your Apple ID.