Dear Maintainer, I had version 1.38-4 installed until I ran apt upgrade today. After the upgrade, mail that originated outside of my system seems to vanish. The mail logs show a normal delivery (by postfix). But kmail shows no sign of the mail. Nor does my android mail client that uses imap to read the same mailbox. I downgraded libidn12 back to 1.38-4 (with NO other changes to packages or configuration) and kmail is working again.
Retracting this report -- it was my confusion. The email I was testing came from an address for which there was a mail rule I'd forgotten about. The mail was safe in a different folder.