If you have a ProBuilder object, and export the object as the 'Obj' format, it doesn't retain the colours of the materials.

1. Create a new project.
2. Install the ProBuilder and UniversalRP packages. (Window -> Package Manager)
3. Create a new Pipeline Asset. (Assets -> Create -> Rendering -> Universal Render Pipeline -> Pipeline Asset)
4. Set as the active Render Pipeline. (Project Settings -> Graphics -> Scriptable Render Pipeline Settings)
5. Open the ProBuilder Window. (Tools -> ProBuilder -> ProBuilder Window)
6. Create a new ProBuilder cube with CMD+K.
7. Make a new material. Set the material's colour to something bright. (i.e. Cyan)
8. Apply this material to the ProBuilder cube.
9. Ensure the cube is selected and click the '+' next to 'Export' in the ProBuilder Window.
10. Export the cube in the 'Obj' format. Save to somewhere in the Project's Assets folder.

Expected Outcome: The exported cube will look exactly the same as the original cube.
Actual Outcome: The exported cube loses all colour.

Reproducible with ProBuilder versions 4.1.2, 4.0.4, 4.0.2 alongside corresponding URP/LWRP versions. (7.1.6, 6.9.0)
Not reproducible with ProBuilder 3.10.1 and LWRP 1.1.11.

Note:
- Tested primarily on Mac. Also appears to occur on Windows.
- Export issues do not appear to occur if you're not using URP.