Thanks for checking in. For the moment, that is considered as being an expected behaviour. The customer marketing opt-in can be set to pre-checked as a default behaviour in the regular checkout service, but the draft order checkout is a special case. Draft orders can be created in such a way that the invoice link will direct the customer to the shipping or payment stage of the checkout, depending on what information has been pre-filled by the merchant. Because the opt-in toggle exists on the first page of the checkout, customers are not given reasonable visibility over their marketing opt-in status unless they manually maneuver back to previous stages of the checkout from where they've started, via prompts or breadcrumb links. Due this reduced visibility, the toggle is automatically disabled instead, so as to prevent instances of customers being automatically opted in against their knowledge. We want to ensure that customers have as much autonomy as possible over any permissions they grant related to marketing and communications.
We are examining this behaviour for future development, but for the moment please consider this as being an intended outcome.