https://docs.mongodb.com/master/release-notes/3.4/#improved-initial-sync

Modified rs.status() output to report on initial sync status and progress.

Turns out (as per DOCS-8598) that to get the initial sync info, you need to run:

db.runCommand({replSetGetStatus: 1, initialSync: 1})

Since rs.status() wraps vanilla command

db.runCommand({ replSetGetStatus: 1 })

, it does not return the initial sync information.