Effect of shading on solar panel
Partial shading can cause massive reductions in power. I looked into this a little, because the best place for a solar panel on my little yacht is under the boom, which often partially shades the panel.
So I measured the short circuit current with and without shading:

2.44A unshaded but only 0.56A with shading covering the entire width of at least one cell.
Interestingly, 1.44A with shading covering only part of the width of any single cell. Still quite a significant loss. My impression was that the axis of the shading mattered; that covering the whole width was more serious than covering the whole height (parallel to the connection wires) but that may have been because this panel uses 1/3 height cells and looking back at my results I have none where I only covered half the height of each cell. Careless experiment design!
Still, it might be interesting to see if your results depended on the orientation of the panel with respect to the grill.