No static content served by Spring Boot when built on Linux

I am currently working on a Kotlin Web Application based on Spring Boot. As I am using continuous integration (GitLab CI), I noticed that the resulting JARs do not work as expected.

When I build the app using mvn package on my Windows development machine and just run it with java -jar ... the app works fine, also executed on Linux.

When I do the same steps on the Linux machine, where the GitLab CI Runner is executed, the resulting JAR is the same, however all static content is not served and results in a 404 error, although the files are present at the right location inside the JAR. This also happens if I copy the Linux built JAR over to my Windows PC and try it there.

Thanks for your help.