In my case I have XnView 2.51.5 installed (as well as XnView MP (x64)) and it is correctly detected (green).
I also updated XnView MP (x64) to 1.6.1 and the situation remains: outdated (red) in the right panel and not installed (black) in the left panel.
I also updated XnView MP (x64) to 1.6.1 and the situation remains: outdated (red) in the right panel and not installed (black) in the left panel.