Troubleshooting Waterfall Chart Issues in Merged Columns

Hey guys, have a quick question. Sometimes when I'm running a waterfall into a merged column, I'll have this little green space in the corner of the cell (without the output). Instead of the waterfall running and merging into one column, it makes me hit the little re-do button in the cell, and then it gives me the output. I've attached a picture to show what i mean. Anyone know why this happens and how to get around it?