Hi vkrukonis21, thanks for asking. We don’t support fetching keys within Custom Ordere Item meta arrays. I’ve got a task to add this in a future Plugin update.
_product_addons is used by the WooCommerce Product Add-ons Plugin, are you using this? If so enter the Custom Product Add-ons names within the Products export type then save those changes for the additional Order Item fields to appear in the Orders export type 🙂