Hi @glitchered (and @Krzysztof.N ),
I have encountered this issue in some different scenarios, not only when using an HDRI. I have prepared a scene isolating the issue. Obviously, this is a test scene with a very certain situation, but I faced this before in real case scenarios (which I can’t share), so a “lab” context is needed, just created in some minutes. It may be quite possible to force other scenarios within more time.
The grid is always there, I think; it’s just related to a combination of your lighting intensities, materials and/or exposures, to increase/decrease the contrast in some way, and expose the grid:
Lab project: Dropbox - BUG_lumen_grid.rar - Simplify your life