You have to use the FastMail app if you want to send from an email alias. That's the only reason I have for not using the native iOS mail app. Please let me know if you have a workaround!
You can send email from your alias with the Mail app, I do it every day.
You need to set it up manually (not use the certificate from fastmail).
After you set up the email address, go to Settings -> Password and accounts -> Tap your account -> Tap you email -> tap the "Email" field and then you can add your aliases tapping the button "Add Another Email"
FWIW fastmail’s web UI is entirely built on top of JMAP. (If you’re curious you can add &debug=true to the URL in your browser to turn off compression and see all the JMAP queries the web UI makes in the inspector). So any 3rd party email clients can support all this stuff on top of JMAP in a standards complaint way if they want to. Anyone who works on native mobile email clients, please add support for aliases and snooze and all these great features via jmap!