Craft Commerce stock not updated when order deleted Is this deliberate, or a bug? Delete an order and the products within the order don't have their stock updated again.

I'm only running Craft Commerce 1.2.1327 but I don't see any mention of it in the update notes to the current version so I'm not sure updating is the answer. I'm also cautious about updating at the moment in case it breaks my third-party payment gateway plugin.
Definitely deliberate.  Inventory reduction when an order is placed is a "convenience" thing.

There are a million reasons why an order might be deleted and only a handful of them would have anything to do with inventory.

But as André points out... you could write a simple plugin to have whatever behavior you wanted around this.