UPDATE: this is kind of a weird behavior …To your first point. Same here. I’ve already reported this issue of not being able to send emails from addresses I re-added … what is very unfortunate.
But I’m also experiencing problems with other addresses in iOS Mail. They suddenly disappear and reappear after a while.
… I’m now able to use the custom email address that I’ve deleted and re-added via beta.icloud.com account settings on my iOS and iPadOS 14 devices for sending email, but I still can’t see/use them in Mail on beta.icloud.com as well as on my devices running the latest iOS 15 beta.