Previously (version unknown, will try to find a working one later), when creating a kmail identity, you can choose a GPG key not matching your email identity. This is useful at least for alias identity using a foo+bar@example.org email address but a foo@example.org GPG identity. In new version (18.12.2-1 arch package), the GPG key drop-down only selects GPG key with a identity matching the given email, and no entry to manually select another key.
Perhaps this is related to https://bugs.kde.org/show_bug.cgi?id=406909
More likely, this was implemented as a feature in https://phabricator.kde.org/T3125 Suggestion for an enhancement: Also allow all keys matching email aliases set in the first identity tab.