Foliage with multiple materials fine in viewport, wrong in standalone

I’ve painted some foliage into my scene using a simple static mesh. The mesh has 2 materials (both simple diffuse colours). The foliage looks great when I play the level in the viewport, but when I play it in standalone mode the materials are replaced by the default “grey squares” texture. Am I doing something wrong?

Changing one of the materials a little and resaving it solved this.