Product Bundles need to consist of individual line-items in the original order for us to be able to differentiate individual items in the return in case you want the customer to be able to return single items from the bundle.

A lot of 3rd party apps create bundles in an order for your Shopify store as a single product. Example: So even though it might look like a bundle of 12 different pairs of socks, the app you are using for bundles only adds all these products combined as a single product in the order. Therefore the customer is only able to return the entire bundle and not individual items.