Pressing Cmd+Enter (or Ctrl+Enter) in the email composer looked like it sent your reply — the composer cleared — but nothing was actually sent, and your draft was gone. The send button worked fine, which is why the problem seemed to come and go depending on how you sent.
The shortcut is now wired up properly: Cmd+Enter sends the reply, exactly like clicking the button. And if the message can't be sent yet — for example, attachments are still uploading — your draft stays put instead of being wiped.
Live for all workspaces — nothing to do on your end.