Turns out the configuration of the spring-boot-maven-plugin was wrong. The configuration <layout>ZIP<layout> was only active for the repackage goal, but not for the build-image goal.

After correctly configuring <layout>ZIP<layout> for the build-image goal, the generated buildpack-image uses the org.springframework.boot.loader.launch.PropertiesLauncher which allows again the use of the LOADER_PATH.