OpenXML SDK border on merged cells only applied to first cell

Question

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:

    <x:borders count="2" xmlns:x="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
      <x:border>
        <x:left />
        <x:right />
        <x:top />
        <x:bottom />
        <x:diagonal />
      </x:border>
      <x:border>
        <x:left />
        <x:right />
        <x:top />
        <x:bottom style="thick">
          <x:color auto="1" />
        </x:bottom>
        <x:diagonal />
      </x:border>
    </x:borders>
    <x:cellXfs xmlns:x="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
      <x:xf fontId="0" />
      <x:xf fontId="1" fillId="2" applyFont="1" applyFill="1" />
      <x:xf borderId="1" applyBorder="1" />
    </x:cellXfs>

I've tried everything, applying the merge before the border, after, etc.

Answer 1

I've figured this out. I was under the impression that if you apply a style to the top left cell in a group of merged cells, then said style will apply to the whole merge. I swear I read some documentation that said that, but this is apparently not the case. What worked is that I applied the border style to each cell in the merge range, not just the first.

It seems weird to me that the way I initially had it is even allowed then, considering the generated output is impossible to reproduce other than through the OpenXML SDK.