Often, province borders can differ from country frontiers. This can occur either because the original data had some small differences due to map simplification routines or because some Map Editor operation. If you plan to transfer regions from one entity to another, you must ensure that country frontiers and province borders match so there’re no remaining artifacts or small land patches left when a polygon operation finishes.
The asset includes an option that ensures that the countries frontiers match the provinces borders. It can be accessed from the Map Editor, gear icon menu:
Note that this option is only available when the editing mode is set to Countries + Provinces and frontiers detail is set to High.
Once the operation finishes, remember to click “Save” to make the changes permanent (it will create a backup) or click “Revert” to discard the changes.