This issue may be indirectly related to bug 1175906 and I'll add a comment to that this week (and switch it to 15.3). It is looking as if Leap 15.3 will be using the SUSE shim instead of the openSUSE shim. In my experiments, I have an EFI boot entry for "opensuse" that is used for 15.3, and an entry for "tumbleweed" that is used for Tumbleweed. This allows me to select which "shim" I am using for booting.