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 use it in tandem with proton, so the quantity of emails going to iCloud are less, however I’ve not experienced this kind off issue with either.
Generally if a service is going to have issues with SL, it’s at the point when you provide the email that it says it’s invalid (likely due to the domain being on a black list to prevent use of the service)
I had one but most likely it was spam (domain is from a job site). Other than that, no other issues between SL and iCloud.