It will probably depend on which GPUs are available at a certain point in time at the quantity desired that meets power constraints, and which chip in a respective performance class Apple can get for the lowest out-the-door price after any of their own processing specs are added on. And I'm not sure if Nvidia currently has a whole lot of newer products that would meet all of the above???
Historically, Apple MBPs have had some issues with certain AMD and Nvidia GPUs - when they have switched from one brand to another on the MBP, this has sometimes happened shortly a MBP GPU issue. It may just be coincidence. But if it is not, since the most recent Nvidia and AMD dGPUs in MacBook Pros both have a pretty solid track record, that may give Apple more incentive to stay with AMD if pricing is favorable, given there isn't a potential PR nightmare on the horizon.