Matplotlib table header

I am having an issue with formatting a table in Matplotlib. I am auto formatting the cells to fit the text size and trying to add a header above the last 4 columns in the table. I merged the cells but am having an issue changing the color. Only half of the cell is changing to light gray. The other half stays white. I am not sure how to change this, any help would be much appreciated.

My attempts at fixing this was to set the cell.set_facecolor(‘lightgrey’)
I tried to set the facecolor to ‘none’ before setting the color to lightgrey but that didn’t work either.