I just tried changing my email on studentaid.gov to a simplelogin alias (using SL is a habit at this point) and I got notifications that emails from it were bounced while trying to verify the email change with sent codes. I looked it up and found a bunch of Reddit posts about issues with SL and iCloud.
I don’t use apple, but I have had this with some other websites. All issues I had were solved by switching to the custom domain setup with SL.