[Jac Mac]

Wonder if its something simple like the assembler not 'cupping' the scraper rails prior to fitting on the piston to check for right way up, or fitting the groove spacer incorrectly ( as used in piston sets with oil ring over piston pin) if it has one.
Another possibility would be the new pistons being delivered without oil drain holes drilled, bit of a rare one but it does happen & if the guy doing the work doesnt notice...