Hi @Matthew.Ivey and @hieblertom ,
Have a look at line 88 of BaseLlightmass.ini
; Approximately clamp the number of surface samples generated to ~30mb (only for Landscape currently)
MaxSurfaceLightSamples=500000
Are we sure that Epic GPULightmass does not generate more SurfaceLightSamples than CPU/LGPU and hence this hit this limit and therefore “build Reflection Captures” silently fails