The safe_secondary_reads_causal_consistency.js test waits for the hangBeforePostMigrationCommitRefresh failpoint to be hit on the donor shard. This only ensures the donor shard has run the _configsvrCommitChunkMigration command against the config server primary and doesn't guarantee the effects of the _configsvrCommitChunkMigration command are visible on all CSRS members. The router may refresh from a node other than the CSRS primary and not see the effects of the _configsvrCommitChunkMigration command.

st.configRS.awaitLastOpCommitted();

prior to running the flushRouterConfig command would ensure the mongos0 process fetches the latest version of the routing table and will always attempt to route the _id:2 insert to shard1.