合并单元格上的OpenXML SDK边框仅应用于第一个单元格

When using the OpenXML SDK to apply a border to a merged cell range, the border is only showing up for the first (top left) cell in the merge range. I have absolutely no idea and this is driving me crazy. Using the productivity tool I can't find any meaningful difference in what I have and something that works as expected. This is my generated XML for the stylesheet:When using the OpenXML SDK to apply a border to