Samsungs camera app force's certain things to save to the internal memory for performance reasons and has been that way since the S6/N5 in 2015. Previous phones forced 4K 60FPS to internal storage, I assume burst and RAW photos are still saved to internal as well.(I don't have the N20)
You most likely need to use a 3rd party camera app if you want to bypass limitations of Samsungs app.