What are the dimensions of the bench?
What's stopping you from adding intermediate material, (stud) to fix and flatten the ply to?
All things sag under its own weight, often the spacers under the stacks of material at the lumb. yd. aren't evenly placed or there may be dips in the pavement it sits on. Thus the material sags to even out load.
If both faces of the sht. are clear count the number of plys, 7 and less is construction grade. The only truly voidless ply I'm aware of is marine grade.
If both faces of the sht. aren't clear you may have AC, it has voids, they may not show up at the perimeter.
Underlayment can have 1 good side and the other plugged, it has voids but they're very small 1/4" or less in diameter and -0- and still warp. Both are and can be used in flooring and construction and don't require perfect flat
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