In BF-45997, there were 5 BFGs since 8/20. The failures fall into two main categories:

• "The shard key provided does not have
  enough cardinality to make the required number of chunks of 100,
  it can only make 51 chunks"
• "Cannot analyze the characteristics of a
  shard key for an empty collection"

The test setup is to insert 10000 documents into an unsharded collection (2 node RS), create an index, and then run analyzeShardKey. The second error is especially concerning, because the insert has a writeConcern of "majority", which should durably write to both nodes.

The most likely explanation is that the writes are durable on the secondary, but are not yet fully applied when analyzeShardKey is run (which uses secondaryPreferred).

A simple solution is to ensure the write is fully replicated to the secondary by adding a call to FixtureHelpers.awaitReplication(db), similar to analyze_shard_key_common_tests.js#L49.