Ingest pipeline: Extract certain line with grok

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser): 2.11.1


currently I try to extract a certain line from a document field with an ingest pipeline.
I want to copy this line into another field.
The line begins with “description:” and the field, in which the string data should be copied, is called description as well.

For that, I built the following ingest pipeline (API command):

PUT _ingest/pipeline/test-pipeline
  "description" : "[...]",
  "processors" : [
      "grok" : {
        "field" : "message",
        "patterns" : ["description:%{GREEDYDATA:description}"]

When I activate the pipeline in my index and start the collection with FileBeat, however there is an error in the FileBeat logs:

Provided Grok expressions do not match field value.

Can anybody help me with the right pattern command? Thank you!