Print masks need to match the Print dimensions which are set on the Print area to output correctly
This information can be found on the Print area list view:
Print Area list view showing the Print dimensions and DPI of the created Print Area.It can also be found if editing the Print area itself:
The Print Area edit page showing the Print Dimensions and DPI

Note that Print Masks only affect the production of the generated Print Output file(s), they have no impact on the Product preview.