Hi,
Aqua Mail Pro user here.
I can confirm the issue of drafts not editable is still present, at least with Fastmail.com
If I start an e-mail in Aqua Mail and save it as Draft, that e-mail looks like a regular e-mail and is not possible to continue to edit it in Fastmail.com
Tested the same with FairEmail Android app and there is no issue, e-mail is recognized properly as a Draft.
My use case: I often start a quick draft on the mobile and then continue on the Desktop to check spelling, add attachments, ... before sending.
Please consider checking and fixing, happy to support with testing.