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Yes, grout is porous. I bet its leaking from elsewhere not just the grout or the front of the seat. Sometimes, water seeps back there, cause the lumber to expand and it does not dry due to lack of air movement.
Water gets behind the vertical wall trying to work its way out back under the tile on the seat. Sometimes a genius will put a nail through the backer board on the flat surface and its all it takes for water to seep inside. Once the lumber is wet enough to expand, it will rip the water proofing more causing more damage.
Is that the only spot that's bad?
Pry it out to have a look, there's nothing to lose now. You have extra tile hanging around?
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