Unable to get multiline to work with Log4j logs

Hey,
I’ve deployed promtail using Helm.
This is from my values.yaml

config:
...
  snippets:
    pipelineStages:
      - cri: {}
      - multiline: # see https://grafana.com/docs/loki/latest/clients/promtail/stages/multiline/
          firstline: '^\d{4}-\d{2}-\d{2} \d{1,2}:\d{2}:\d{2},\d{3}'
          max_wait_time: 3s
    extraRelabelConfigs:
      - action: replace
...

I’m trying to match log lines looking like this:

2021-09-03 12:05:53,626 INFO  org.apache.flink.kubernetes.highavailability.KubernetesHaServices [] - Clean up the high availability data for job 00000000000000000000000000000000.
2021-09-03 12:05:53,646 INFO  org.apache.flink.kubernetes.highavailability.KubernetesHaServices [] - Finished cleaning up the high availability data for job 00000000000000000000000000000000.
2021-09-03 12:05:53,718 INFO  org.apache.flink.client.deployment.application.ApplicationDispatcherBootstrap [] - Application FAILED:
java.util.concurrent.CompletionException: org.apache.flink.client.deployment.application.UnsuccessfulExecutionException: Application Status: FAILED
	at org.apache.flink.client.deployment.application.ApplicationDispatcherBootstrap.lambda$unwrapJobResultException$5(ApplicationDispatcherBootstrap.java:345) ~[flink-dist_2.11-1.15-SNAPSHOT.jar:1.15-SNAPSHOT]

The promtail pods are not logging any errors that mention multiline.