Weird, I could have sworn that I had responded here already. Anyway:

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Could you speak a bit more about the seal? If I assume that the underfloor siphon and ventilation work, then the portion of the pipe that could be affected by a leak in the seal would only contain a limited amount of smelly air, right?

This toilet is stepped on regularly, for example to turn off the gas outside. I also did install a new toilet seat, which doesn't involve drilling, but a fair bit of shaking and movement - could these things have unsettled a seal?