Hello,
You can use pre tags to keep extra spaces, but please note that you may need to set new styles to avoid default ones:
$docx->embedHTML('<p style="font-size: 16px; font-family: Calibri;">Sentence begins</p>');
$docx->embedHTML('<pre style="font-size: 16px; font-family: Calibri;"> and a previous sentence continues </pre>');
Other approach would be using a WordFragment and adding spaces with addText:
$html = new WordFragment($docx);
$html->embedHTML('<p>and a previous sentence continues</p>');
$text = array();
$text[] = array('text' => ' ');
$text[] = $html;
$text[] = array('text' => ' ');
$docx->addText($text);
Regards.