Lighting samples are missing in some areas:
It works fine in other areas with the same settings.
Any idea of what might cause this issue?
Make sure you’ve got a Lightmass Importance volume around your playable area. You can have multiple ones in your level too, so it doesn’t have to be one all encompassing one. There was a change in 4.15 that I think causes this now, which is expected:
Volume lighting samples are allowed outside of the importance volume if their influence affects the volume. Fixes black indirect lighting on movable components in maps with small importance volumes. Volume lighting samples placed on surfaces use a radius that covers the layer height spacing, which prevents an uncovered region between layers.
Just add you’re lightmass importance volume and rebuild lighitng. I think this should solve your issue.
There was an importance volume, but it was rather big, so I replaced it with a small one to show the issue:
This happens to me even in areas covered by both the Lightmass Importance Volume and Lightmass Character Detail Volume. It appears as if it simply refuses to place probes above certain meshes, as if the floor mesh has been extruded upward to the sky and cut them out.