The reason for this we can explain with an example. Let's say sheet A is sales orders, and sheet B is customers. When a sales order is entered by user Martha Stewart, her address information is populated with link & load on your sales order. Let's say Martha changed her address. Her previous sales order would still be under her previous address for the records, but new sales orders will be saved with her new address.
We do understand this might not be how you use link & load, and that you will need to repopulate the loaded fields in all your previous records in sheet A.
To do so, navigate to the Form page Design Mode, and click on Sheet Linking Tools to see a list of sheets that are linked with Link & Load under the category. Click on the gear icon next to the appropriate sheet.
A prompt will show you the option for dynamic filtering above, and the option for repopulating below, that gives you the option to synchronize a single set of linked fields with the updated source value for all records, or all of your linked fields on this form.
After choosing your option, a prompt will state that loading has started. This means that your loaded fields are reloading in the background, so you may now close this window and continue using your database while your data is synchronizing.