FTS is designed for flight termination, not sinking the booster after ocean soft touchdown. So if FTS detonation fails to terminate the flight sufficiently quickly, that counts as an FTS failure which FAA will require to be investigated as a safety mishap, grounding the launch vehicle in the meantime. But if FTS fails to sink the booster after ocean soft touchdown, it wasn’t designed to do that anyway, so I don’t think the FAA will consider it a “failure”. If the flight plan lists this outcome as a possibility, and SpaceX has a backup plan to respond to it, it isn’t a failure as far as the FAA is concerned.