The only thing the dual native ISO auto setting does is that it opens up the whole range and automatically change the circuit when you reach 4000. That is how it works on the S5. I cant change that.
Auto ISO and dual native ISO auto setting is to different things. I'll try to explain the best I can 🙂
When you are using manual ISO:
Think of the dual ISO like a manual gearbox with a high and a low setting. The low setting is gear 1 to 3 and the high setting is 4 to 6. If you use the low setting, you can only use 1, 2 and 3 (in the camera that is 640-3200) and if set to high you can only use 4, 5 and 6 (in the camera that is 4000-51200). If you use the auto (in camera that is dual native ISO auto setting) you can use 1, 2, 3, 4, 5 or 6 (in the camera that is 640-51200).
But the camera also has a auto ISO setting which make the manual gearbox automatic but you still have the choice of low, high and auto dual ISO settings.
If you use auto ISO in the low setting it will use 640-3200 based on your exposure settings, it will never go higher than 3200. The same with the high setting but it will only use 4000-51200.
In the dual ISO auto setting it would use the whole range 640-51200 based on your exposure settings.
The camera doesn't care that 4000 is cleaner than 3200, it only choose the ISO to match your exposure settings.
The camera is recording at the value you set, 800 is 800, 1600 is 1600 and so on. It's the same as a camera without dual native ISO.
Check this video from DP Journey, he shows how the it works in the S5 and how to get the best noise performance.
His conclusion for minimal noise in the image is to use 640-1250 or 4000-6400. Check the charts in the video.
I hope this helps 🙂