SERVER-84796 disabled the single write shard commit optimization for updates that change a document's shard key value and move it across shards. It does so by setting a flag on the TransactionRouter before running the update shard key transaction. That flag is never unset though, so future transactions using that TransactionRouter (ie same logical session and same mongos instance) cannot use the optimization. Instead, we should unset the flag when resetting the TransactionRouter state here.