A quick way to isolate the cause of the behavior is to determine if a custom Status is used in the Opportunity status field:
1. Navigate to Setup > Sales > Customer Statuses.
If this is the case then the cause of the behavior is most likely the Probability that has been set for the custom Status. NetSuite automatically changes the Document Status to Closed - Lost if the Probability of the Customer/Opportunity Status used on the Opportunity is below 1%.