After cleaning my old pump filter of deposits I noticed that a seemingly significant number of the small rectangles which make up the filter grid are not open but are filled in with plastic. Is this typical of the manufacturing process for the filters? Or could these have become filled in through long use?

If I purchase a new filter such as Whirlpool WP99001791 Pump Filter - AppliancePartsPros.com should I expect all grid rectangles to be open?

Thank you,
David