When trying to calculate vertical grids (internal building lighting to external lighting grid) I'm getting a set of results that look spurious, potentially as though its reflected light from something else (that wouldn't exist)

Figure 1

This version (figure 1)has a grid to the eastern boundary showing 0.51 but presents a weird pattern along the grid.

If i add a grid behind it, it changes the numerical value (Figure 2) and grid pattern to present in a more realistic pattern (results in Figure 3).

Figure 2

Figure 3

why is this? is it a bug?