AppendProcessorTests.testAppendingUniqueValueToScalar fails on casting

[pgomulka]

Build scan:
https://gradle-enterprise.elastic.co/s/tjxkfpkfk4p7s

Repro line:
./gradlew ':modules:ingest-common:test' --tests "org.elasticsearch.ingest.common.AppendProcessorTests.testAppendingUniqueValueToScalar" -Dtests.seed=7ADBC57E6F3321EC -Dtests.security.manager=true -Dtests.locale=mk-MK -Dtests.timezone=Greenwich -Druntime.java=11

Reproduces locally?:
yes

Applicable branches:
master (have not tested others)

Failure history:

Failure excerpt:

> java.lang.IllegalArgumentException: field [Yb.񂉙񋢲.aQ.BFkPjlFm.mt7F4aWi] of type [java.lang.String] cannot be cast to [java.util.List]
        at __randomizedtesting.SeedInfo.seed([7ADBC57E6F3321EC:5156B1CAEC68E9FE]:0)
        at org.elasticsearch.ingest.IngestDocument.cast(IngestDocument.java:638)
        at org.elasticsearch.ingest.IngestDocument.getFieldValue(IngestDocument.java:115)
        at org.elasticsearch.ingest.common.AppendProcessorTests.testAppendingUniqueValueToScalar(AppendProcessorTests.java:186)

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Ingest)