Be aware that shadowing with GPULM in general is broken in 5.3: See Epic's GPUlightmass
Could this be your issue?
If you can, test in 5.1.1, which is the last good working engine with GPU Lightmass.
Be aware that shadowing with GPULM in general is broken in 5.3: See Epic's GPUlightmass
Could this be your issue?
If you can, test in 5.1.1, which is the last good working engine with GPU Lightmass.