The test with Gpg4win 4.3.1 (using GnuPG 2.4) seems to indicate that:
- gpg didn't update the trustdb automatically after importing the extended trusted certificate.
- gpg updated the trustdb automatically after deleting and re-importing the expired trusted certificate, but Kleopatra still showed the certificates signed by the trusted certificate as "certified". This could be a bug in the trustdb calculation (note the log message "Schlüssel C5D6C919005F36A4 ist als ultimativ vertrauenswürdig gekennzeichnet" which could indicate that gpg treats the key as valid although it's expired). On the other hand, my test with GnuPG 2.4 on Linux doesn't reproduce this problem.