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.
Definitely a workaround, but I wish we didn't have to do this every time. It seems like deleting and recreating the legend will fix the issue (at least sometimes?), but I can't tell what triggers it to happen in the first place or what triggers it again after creating a new legend. Seems like a fresh, new bug for Pro; I don't recall dealing with legend issues as much in ArcMap, even if the GUI was a little confusing.