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.
Depending how deep you are into using the service, this might be an indicator to start shopping around for other options, as there are some that provide multiple domains and unlimited aliases for the cool price of $0 versus whatever Proton charges you…
…Especially if iCloud makes the other side of the equation difficult.