I am using Kubuntu 18.04 with IMAP e-mail. I installed Kube via Flatpak yesterday.
The folder tree and the messages themselves seem to be working fine.
And I can also see and download any attachments in the Inbox.
However, I cannot see, and therefore cannot save, any attachments in the Sent folder.
When I move a message with an attachment (attachment shown in Sent folder in KMail but not in Sent folder in Kube) to Trash, the attachment is also not shown and therefore cannot be saved.
All Kube Sent messages lack an attachment, even though the attachments are on the e-mail server and in KMail.