not sure if it's a bug or I'm doing something wrong but when I export any Subtool with Polypaint using FBX it will set the color of vertex ID 0 to all other vertices. it still exports as expected using OBJ.
this happens on all meshes whether it's Dynamesh, Zremesh, with and without UVs.
the red is vertex ID 0:
the export settings:
in Maya:
Arnold render:
Marmoset viewport: