I'm not surprised there's no reference to the TOC8 Style in that dialogue. It has nothing to do with the heading, per se - it's a Table of Contents Style. Go to your Table of Contents and click on the paragraph break at the end of the 'Summary' entry, then:
• modify the alignment using the Ruler;
• right-click on the paragraph break; and
• choose Styles>Update TOC8 to match selection.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]