After running an existing map, if you click Connect on the Target map to reconnect to the target connector, then the following pop-up message may be is displayed:

This happens when the target file exists and potentially has a different schema.

If you click Yes, then all the existing target records are removed. Since the existing events are associated to the existing target records, the events are also removed and are not re-applied after connecting to the target. You must rebuild them because the schema will be different. If you click No, then the existing schema remains the same so the events will be maintained. In this case, however, you may want to change the Schema Mismatch property to Use Connection, Match by Name, do a refresh to pick up any new fields, and remove fields that no longer exist.