This may actually be intentional, at the very least I can see how it would happen - product cost is an inventory item concern and not product, so inventory_item webhooks may be required to keep up with these particular updates.
With the above being said, however, I'll be sure to check with our developers here to get you a more definite answer, because I know updating the cost from the product page still does fire the webhook. I'll post back in here with more information once I'm able!
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