1.) Could be. Could be alot of things.
2.) Well the plumber screwed up. No doubt there. He has liability insurance, doesn't he? The cupped boards is telling us, there is moisture under the floor, and has nothing to do with the finish.
Time to remove some flooring to let it all dry out(the subfloor)
If they sand the cupped boards now, it will eventually dry out (it takes time) and the floor boards that were sanded when still wet and cupped, will then be just the opposite, or what we call crowned.
There is a chance, leaving it wet between the flooring and subfloor, will also grow mold.
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