I'm having this same bug as well. I haven't found a way to make the legend export properly, however, I usually copy and paste the legend and then convert the pasted one to graphics. This way, if my legend needs to change in the figure, I still have one created that will dynamically change with the layers on the map, but the one that is converted to graphics does not export the image service layers. It's annoying when I forget to do this and realize after exporting a 60 page map series that I didn't convert to graphics.