Why it's flagged. A paragraph has its own spacing, indent or alignment instead of getting it from a style. The build uses the book's spacing instead, so hand-set values don't carry over.
How serious. Just noting.
How to fix it. Usually nothing to do. If the spacing meant something, in any editor:
- Clear the hand-set spacing or indent (your editor's "clear formatting" or paragraph reset).
- Apply the paragraph style that matches what the text is: Block Quote, Verse, Epigraph and so on.
- Don't use extra Enter presses or spaces to push text around.
When to leave it. If you can't tell any difference on the page, leave it. Build a proof to check.