In the current ArcGIS Explorer release (20.0.1, build 2647), raster layers can be included in a mobile map package (mmpk) authored in ArcGIS Pro, but the symbology of the raster layers in the source ArcGIS Pro project is not honored. Instead, they revert to what appears to be a default grayscale stretch symbology. The source map in ArcGIS Pro looks like this:
Here's what the same area looks like in Explorer:
Not bad, but not nearly as nice as the original. It would be really nice if Explorer could be enhanced to honor the original ArcGIS Pro symbology. The desired functionality would be especially useful with digital elevation rasters and the like, where the important information is encoded in the pixel values.
For those who would like to play with this data: