Flink testParDoRequiresStableInput flaky

https://ci-beam.apache.org/job/beam_PreCommit_Java_Commit/20253/

org.apache.beam.runners.flink.FlinkRequiresStableInputTest.testParDoRequiresStableInput

java.util.concurrent.ExecutionException: java.util.concurrent.CompletionException: org.apache.flink.runtime.checkpoint.CheckpointException: Checkpoint triggering task Source: Impulse -\> CreatePCollectionOfOneValue/Read(CreateSource)/ParDo(OutputSingleSource)/ParMultiDo(OutputSingleSource) -\> CreatePCollectionOfOneValue/Read(CreateSource)/ParDo(BoundedSourceAsSDFWrapper)/ParMultiDo(BoundedSourceAsSDFWrapper)/Pair with initial restriction/ParMultiDo(PairWithRestriction) -\> CreatePCollectionOfOneValue/Read(CreateSource)/ParDo(BoundedSourceAsSDFWrapper)/ParMultiDo(BoundedSourceAsSDFWrapper)/Split restriction/ParMultiDo(SplitRestriction) -\> CreatePCollectionOfOneValue/Read(CreateSource)/ParDo(BoundedSourceAsSDFWrapper)/ParMultiDo(BoundedSourceAsSDFWrapper)/Explode windows/ParMultiDo(ExplodeWindows) -\> CreatePCollectionOfOneValue/Read(CreateSource)/ParDo(BoundedSourceAsSDFWrapper)/ParMultiDo(BoundedSourceAsSDFWrapper)/Assign unique key/AddKeys/Map/ParMultiDo(Anonymous) -\> ToKeyedWorkItem (1/1) of job 7bbb425ba325dbc1dc4d3cdf1c8b88f9 is not being executed at the moment. Aborting checkpoint. Failure reason: Not all required tasks are currently running.

Imported from Jira [BEAM-13575](https://issues.apache.org/jira/browse/BEAM-13575). Original Jira may contain additional context.
Reported by: ibzib.

Start with the Apache Beam CI job linked in the issue and reproduce FlinkRequiresStableInputTest.testParDoRequiresStableInput. Trace the reported checkpoint failure and determine why the test intermittently reaches a non-running task. Done means the test no longer flakes and the Java pre-commit job passes reliably.

Written by the indexing model from the issue text.