How to get rect source texture to work?

Hello, I’m using a rect light with static mobility, and a Source Texture to try and vary the lighting, however, it seems to do nothing and I’ve tried half a dozen textures. I also set the mip level to Blur5 as suggested in the docs.

Any tips on how to get Source Texture to work? I’m using GPU Lightmass, and as I’m writing this I’m thinking that might be the problem, I’ll give CPU Lightmass a try.