When you reload the map data in Surfer, you reload the file and replace all properties with the properties in the file. If the line style (or any other property) is not defined in the original base map file, then when it is reloaded the properties will revert back to the defaults.

To add your vote to the request on the suggestion file to have the option to keep the properties that have been changed since loading the file, please email surfersupport@goldensoftware.com.