Since global fields store the same data for all your records, they won't change when you create a new record. You'll need to clear this field as part of your new record operation or clear it as the last step of a script where you used that data in the global field in the first place.
you can use Set Field [YourTale::YourGlobalField ; "" ] To clear the global field in a script.
Example 1: If the global field is used in a script to find records, you can add this step to the end of the script to clear the field after you have found the records.
Example 2: If you put a button on your layout for creating new records, it could use this script:
Set Field [YourTale::YourGlobalField ; "" ]
Example 3: If you have FileMaker Advanced, you can add a custom menu for your layout that performs the script from example 2 whenever you select New Record from the Records menu or press the keyboard shortcut for creating a new record.