Huh, this is surprising. I remember reporting issues with GPULM volumetric lightmap creating invalid samples, causing high contrast probes with burnt black areas on UDN over a year ago. I was told that this was due to GPULM not supporting bent normal skylight illumination. That answer sounded odd to me, but I never disputed it. It’s crazy to believe that bug survived for such a long time, but I am glad it’s on the radar finally.
For me, the GPULM produced both faster and more accurate surface lightmaps, but I’ve almost never ran into a level where the volumetric lightmap result wasn’t broken/corrupted.