I don’t know - if both parents are U.S. citizens, I think historically the child was considered “natural born.” You’ve had some American families abroad (vacation, whatever) when a birth occurred and there was no issue about the baby being natural born.
There was never any doubt about Citizenship, but there has always been doubt as to NBC status. I was in the Military 58 years ago and this came up in discussions frequently, I never new anyone that doubted that if we had children overseas, they would not be eligible to be President.
Think of all the Children our men left in various outposts.