Yep, OSB flooring. They can say what they want, but it just doesn't cut the mustard. I would suspect the flush valve leaking more than the seal, as the seal is for gas, to keep the "stink" down. The horn on the bottom of the stool should dump waste down the pipe without the seal, unless there is a blockage downstream causing a back up. Of course, there is always a chance the stool is cracked.
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