The pump basket strainers that I am familiar with are to coarse to catch sand, it would pass right thru on to the pump. Exactly where are you catching sand? In the stainer basket or below it in the body of the pump basket?
Most likely, it is being picked up off the pool floor by your vacuum or suction side cleaner and, because of it's weight not all of it makes it thru the pump to travel into the filter.
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