Not sure what you mean; active sprints need to have a start date and end date to allow them to timebox.
When you say you cloned the sprint value box - do you mean you cloned an issue which belonged to a sprint?
Do you perhaps have custom fields for start / end date of an issue? Usually start / end dates refer to the sprint itself rather than one specific issue.
Perhaps a screenshot would be beneficial to show which fields you are referring to?
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