"Send to Substance" doesn't retain material IDs?

Title says it all really. With the Substance plugin, sending a mesh to Substance makes it have 1 “texture set” in Substance called “worldgridmaterial”.
If I import the exact same mesh it has all the correct material IDs, same as if I look at it in UE4, so somewhere in the middle of sending it from UE4 to Substance the material IDs are lost.