The length of the duct run might be a variable to look at? But from my experience most, if not all, of the low-to-mid range domestic extractors seem to be decorative items.
I tested it with the extractor duct disconnected and saw the same bahaviour.
I suspect that your experience, and subsequent asessment, may be the correct answer. I live in hope of a second opinion.
@Aroogah started
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The length of the duct run might be a variable to look at?
But from my experience most, if not all, of the low-to-mid range domestic extractors seem to be decorative items.