There is an issue with formatted inline strings.

Cell xml:

<c s="1" t="inlineStr" r="A1">
  <is>
    <r>
      <t>Regular </t>
    </r>
    <r>
      <rPr>
        <b/>
      </rPr>
      <t>Bold</t>
    </r>
  </is>
</c>

Expected csv value: Regular Bold
Actual csv value: Regular

The proposed PR fixes this behavior.
I've added a test for this case and make sure all the other tests pass.