Incorrect color on bounced lighting

I’m getting some unexpected color results from Lightmass so I recreated my issue in an empty project. I’m running on Windows, stock engine version 4.13. Note the orange color bouncing onto the underside of this sphere, when I would instead expect green:

I turned on the show flag for Volume Lighting Samples so that’s where the little squares come from. The scene is lit with a single Stationary directional light, lighting build quality is Production, and lightmaps are relatively dense for testing purposes. The floor and sphere are the only objects in the scene, and are default engine primitives (not BSP). I have not added any postprocessing.

The full scene looks like this:

I suspect the issue is related to my floor material. I’m coloring it with a texture that is projected on the XY plane:

I ran into this because we use this projection method to alter the color of our grass and terrain on a large scale across our landscape. The color map is very small (64x64). Note that if I just use a constant color instead of a texture projection, my results look as I’d expect them to.

Any advice on how to proceed? Could this be something I’m doing wrong with my material, or a misunderstanding of how Lightmass works? Without debugging Lightmass directly I’m unsure how to proceed, and I don’t know of a great work-around to create the very nice smooth color gradations that this approach offers for our landscapes.

Thanks!