Interesting story. The decomm process would not complete until all blocks have at least 1 good replica on other DNs. (good replica = replicas that are not stale and on a DataNode that is not being decommissioned or already decommissioned) DirectoryScanner in a DataNode scans the entire directory, reconciling inconsistency between in-memory block map and on-disk replica, so it would eventually pick up the added replica, just a matter of time.
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