During startup StandbyNode reports that it needs additional X blocks to reach the threshold 1.0000. Where X is changing up and down.
This is because with fast tailing SBN adds new blocks from edits while DNs have not reported replicas yet. Being in SafeMode SBN counts new blocks towards the threshold and can stay in SafeMode for a long time.
By design, the purpose of startup SafeMode is to disallow modifications of the namespace and blocks map until all DN replicas are reported.