Comment # 6 on bug 1184326 from Michael Andres

Sorry for the confusion. If found a note in the Leap 15.3 release notes:

  https://doc.opensuse.org/release-notes/x86_64/openSUSE/Leap/15.3/

  2.1 Seamless upgrade from openSUSE Leap 15.2

  Unlike 15.2, the default installation of openSUSE Leap 15.3 contains the
  majority of rpms from SUSE Linux Enterprise Server. These rpms are signed by
  SUSE LLC instead of using the openSUSE key....


Looks like Leap 15.3 does not yet pay attention to the key import. I'll try
raise this issue...