Hmm! I guess I can try to add this. The thing is, it may be impossible for it to be like one frame. The other map needs to be loaded, needs to drawn the tilemap etc. From one map to another, it always have some loading. Sometimes is too fast that we can notice, but depending on the map, it could take a while. That is why usually it has a fade transition.
You can try use the "None" transition type to see if it works the way you want.
You can also try to use "Fade" with a duration of 1 frame.
Another attempt would be to try the Zoom. Set the Zoom Target Value to 1, Zoom Duration to 1, Fade duration to 1.
Let me know if any of that works.