Within Uptics, if your prospects are enrolled via a SmartList or Manually in the existing sequence, even though they have been unenrolled manually, they are yet a part of the SmartList/existing sequence and will thus be auto-enrolled in a new step.
Only if the prospects in your existing sequence have been unenrolled due to a sequence setting criteria for example: 'Unenroll if Replies to this Sequence' in this scenario, these unenrolled contacts won't re-enroll back into the sequence.