It seems like the ScummVM engine is actually less buggy than the PC version. The reason the dragon was not in the square was due to the last puzzle not being completed.
In the old buggy version this was circumvented by walking to the square where the dragon stood, even though she apparently should not, and feeding the small swamp dragon a lit firecracker. This would trigger the end scene.
The game seems to work as intended.

I'll go back to attempting to force the soot bug again. I've yet to be successful, though, so it seems to be a random game bug.