When viewing messages in your Sent or Drafts folders, you can choose whether to include sent messages and drafts that have been moved to a different folder (i.e. Archive, Spam, Trash).
By default, moved messages in the Sent and Drafts folders are hidden from view. To enable this feature:
1. Go to Settings → Go to settings → Proton Mail → Messages and composing → Messages → Keep messages in Sent/Drafts and toggle the switch on.