Using a custom ConfigMap with Quarkus with packing-type 'fast-jar' does not work

Question

The problem

After changing the quarkus.package.type to fast-jar, the application instances on our Kubernetes cluster could no longer read a mounted ConfigMap.

2022-02-17 14:39:45,155 WARN [com.example.MyStartupService] (main)
    Config param is not defined and might be missing: foo.bars

Locally there is no problem with the configuration, but we also do not mount it - just have a static application.yml. Before this change everything worked fine, we can directly track it to the quarkus.package.type.

Our Setup

First and foremost: We do not use a application.properties, but a application.yml (utilizing quarkus-config-yaml).

We are using Quarkus 2.7.1.Final.

Java Code

@ConfigMapping(prefix = CONFIG_PREFIX)
public interface FooBarConfigMapping {

    String CONFIG_PREFIX = "foo";
    String CONFIG_NAME = "bars";
    String CONFIG_PARAM = String.join(".", CONFIG_PREFIX, CONFIG_NAME);

    @WithName(CONFIG_NAME)
    List<BarConfig> barConfigurations();

    interface BarConfig {
        @WithName("name")
        String name();
    }
}

@ApplicationScoped
public class MyStartupService {

    @Inject
    FooBarConfigMapping fooBarConfigMapping;

    @Transactional
    void startup(@Observes @Priority(2501) StartupEvent event) {
        if (fooBarConfigMapping.barConfigurations().isEmpty()) {
            LOGGER.debug("Config param is not defined and might be missing: " + FooBarConfigMapping.CONFIG_PARAM);
        }

        // do something with fooBarConfigMapping.barConfigurations()
    }
}

Deployment

Our application-config.yaml (ConfigMap):

apiVersion: v1
kind: ConfigMap
metadata:
  name: application-configmap
  # ...
data:
  application.yml: |
    foo:
      bars:
        -
          name: A
        -
          name: B
        -
          name: C

Our deployment.yaml (Deployment):

apiVersion: apps/v1
kind: Deployment
metadata:
  # ...
spec:
  # ...
  template:
    # ...
    spec:
      # ...
      containers:
          # ...
          volumeMounts:
            - name: application-config
              mountPath: /config
      volumes:
        - name: application-config
          configMap:
            name: application-configmap

Misc

1. I can't find any good information what might have changed between legacy-jar and fast-jar i.r.t. reading out the config. Maybe it is related to the different structures (as shown here). For me this might be the likeliest source of the error.

   1.1. I tried to change the mount-path from /config to /usr/local/apps/config (following this SmallRye ConfigMap example), but to no avail.

   1.2. I also tried to rename the mounted file from application.yml to application.yaml, but same outcome.