Drive should be considered suspect - I’d personally get a replacement drive & start burning in the replacement, then after the 10,000 hours of blackblocks & smart long tests, I’d replace the suspect drive (if replacement passed all tests).
Drives have manufacturer set thresholds for failure - technically the drive has not reached the threshold & has passed the test, therefor it is ‘okay’. Think of it as going to a doctor who passed their exams with 51% (or whatever the threshold for failure is depending on school) - technically everything is fine, but consider a replacement 
Depending on your risk tolerance/budget for replacement parts you could just let the drive ride & monitor the dead sectors. If they start increasing in quantity then replace asap, otherwise the failed sectors have been marked by the drive & will not be used for new data.
As for why no data lost, either those sectors didn’t have any when they were marked as failed, or the drive was able to copy over the data to working sectors before blacklisting the failed ones successfully.
tldr; have a replacement ready & either swap asap or monitor & expect to swap when failed sectors increase