I am assuming you have a roof vent pipe on your tank.
As long as the bowl holds water, your blade seal is fine. That wouldn't cause an odor until all the water leaked out, then gasses could escape. The water itself is the barrier. You wouldn't see "bubbles" unless there was positive pressure in the tank, as in no vent pipe.
I would tend to suspect the seal of the toilet to the tank/floor would be the culprit.
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