AssertionError: Missing assignment for [[index-0][4]] during snapshot

[DaveCTurner]

Elasticsearch version (bin/elasticsearch --version): https://github.com/DaveCTurner/elasticsearch/tree/2021-07-16-snapshot-finalization-order-testing-WIP which includes #75362 plus some extra stress tests.

After many iterations of SnapshotStressTestsIT on the linked branch we tripped an assertion in SnapshotsService$RemoveSnapshotDeletionAndContinueTask, see below.

Provide logs (if relevant):

        java.lang.AssertionError: Missing assignment for [[index-0][4]]
            at __randomizedtesting.SeedInfo.seed([A599955E699077CA]:0)
            at org.elasticsearch.snapshots.SnapshotsService$RemoveSnapshotDeletionAndContinueTask.updatedSnapshotsInProgress(SnapshotsService.java:2662)
            at org.elasticsearch.snapshots.SnapshotsService$RemoveSnapshotDeletionAndContinueTask.execute(SnapshotsService.java:2502)
            at org.elasticsearch.cluster.ClusterStateUpdateTask.execute(ClusterStateUpdateTask.java:48)
            at org.elasticsearch.cluster.service.MasterService.executeTasks(MasterService.java:701)
            at org.elasticsearch.cluster.service.MasterService.calculateTaskOutputs(MasterService.java:323)
            at org.elasticsearch.cluster.service.MasterService.runTasks(MasterService.java:218)
            at org.elasticsearch.cluster.service.MasterService$Batcher.run(MasterService.java:155)
            at org.elasticsearch.cluster.service.TaskBatcher.runIfNotProcessed(TaskBatcher.java:139)
            at org.elasticsearch.cluster.service.TaskBatcher$BatchedTask.run(TaskBatcher.java:177)
            at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:678)
            at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:259)
            at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:222)
            at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
            at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
            at java.base/java.lang.Thread.run(Thread.java:831)

testoutput-1626444817.tar.gz

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)