As mentioned, your screen isn't shifting. Its the lighting conditions that make the screen look different. For example, in bright sunlight, your screen may look less warm, true tone or not. The screen itself isn't changing in this instance, it's just the lighting conditions you are in. Weather or not you just switched off true tone, your screen would have looked the same had you left true tone off and simply moved from an indoor location to the outside. This has nothing to do with True tone or the sensors on the phone.
In order to actually compare, you need a reference phone to compare to. If your phone looks less warm in sunlight, the other phone will look even cooler. Trying to compare the same screen in different lighting to itself will not help you with this issue.