Hello there, I have been with my XT2 for something like 2 months now and I have also noticed what you mention and is shown in a video here.
I thought it was a EVF Brightness inaccuracy and have played around it with different values, -1, -2. even -3 , on the outside and with bright daylight.
I thought it could be that but.... as per your post ...looks like we are now looking for something different.
In the interiors... looks like it is better and there is not such big difference but still not as good as on XT1.
That´s my personal experience.