Unfortunately it seems that having 5 copies of the mesh painted as different foliage types did not work. I made sure the equivalent amount of trees were painted (around 31k) but the only thing that has changed is that I now get 5 sets of warnings instead of one:
Note that all the warnings refer to the same InstancedFoliageActor - this further leads me to believe that all foliage is being lumped together into one actor for some strange reason.