It appears you have an image that has been converted from RGB to a colormap. This is a process that some companies use as a form of compression. With a colormap it can not be used as input to many analytics functions. What you can do is use Imagery/Raster Function and apply the 'Colormap to RGB' function. This will create a new layer that now acts as an RGB image that can be used for different analysis. It has the advantage that no new image is created
The export raster with 'use renderer' and 'force RGB' on should also work. The apparent changes in intensity you are seeing are not changes in values, but the fact that when you export an image the system computes stats on the image. When you add the image it by defaults stretches the image based on the stats. In the appearance tab set stretch to None to see the original values.