I just did some testing, and it seems the biggest factor causing this pixelated lighting (square sparkle shapes in the shadowed areas) is from geo which is intersecting and especially having the full face bleeding out from the intersection point (likely the ceiling in OP’s example). This catches indirect/bounced lighting rays from the directional light, or other large light which touches the many surfaces.
The fix has been to delete stacked faces on geo. There should be no top face or thickness to that shower wall tile’s wall. The ceiling should also stick out as short as possible past the shower wall, if the ceiling and walls are not welded together. If there is a hamburger model, the interior buns and paddy would all have their interior faces deleted (the portions un-scene). Better yet, the mesh should be built air-tight with no interior faces, for optimization. It seems that lightmass, especially with directional lights, rewards artists for building their art very cleanly (i.e. welded together, no long triangle slivers, verts un-welded next to each other, submerged faces, etc.).
Shout-out to the Not Yet Lightmap Tools plugin for helping me troubleshoot this issue 50x faster.