Yes, this we already determined earlier in this thread. It is RH-67878 Attempts to add more decals after first fail. This is on the todo list of @andy and looks like it is currently slated for Rhino 8
I see that assigning a decal via code doesn’t work at all in Rhino 8, logged as: RH-71351 Not possible to add decal via code on the list of @andy