When you import a Magnet Field MJF file, the "Point Code & Attributes" screen has an option by default ticked... MJF and TSJ Files > Import Linework. The linework in MField is limited compared to the line styles attributed to the Library Survey codes. So if you leave that option ticked, you just get the MField line styles, unticked you get no lines. Solution: Create a Default Job that loads when you create a new job. Select Survey > Global Codes and transfer to current Project. Import the MJF file and Untick the "Import Linework" option. It then associates the field codes with the Project Codes and the correct linestyles are applied.
Main Solution: when you have transferred the Global Codes to the Default Job and saved to your Library, retain in the Default Job the codes so that when you create a new Project, the saved Project Codes are pre-loaded.