Having sand in your backwash is the reason why you probably don't have enough sand in your filter. In other words, low sand doesn't cause backwash leakage: backwash leakage causes low sand.
And . . . backwash leakage is commonly caused by the oversized pump / undersized filter combinations so typical of metal sided above-ground pool installations.
BTW, please don't multipost again -- you had posted 4 versions of the same question.
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