https://www.voanews.com/usa/diplomats-its-common-politically-appointed-ambassadors-resign
... State Department spokesman John Kirby said Friday, and he confirmed that ambassadors who are career members of the U.S. Foreign Service — about 70 percent of the top American envoys abroad — were not asked to offer their resignations...
That is the choice of the incoming President. Unless directed otherwise, career personnel submit their resignations. I have seen it done that way as well.