Hello,
First line indentation isn't a numbering property but a paragraph one.
phpdocx doesn't support applying first line indentation to li tags through CSS, only paragraph (p) tags and list-style-type: none are supported. You can accomplish it using HTML Extended available in Premium licenses with the data-listppr property:
$latinListOptions = array();
$latinListOptions[0]['type'] = 'upperLetter';
$latinListOptions[0]['format'] = '%1.';
$latinListOptions[0]['left'] = 1440;
$latinListOptions[0]['hanging'] = 720;
$latinListOptions[1]['type'] = 'decimal';
$latinListOptions[1]['format'] = '%2.';
$latinListOptions[1]['left'] = 2160;
$latinListOptions[1]['hanging'] = 720;
$docx->createListStyle('mylist', $latinListOptions);
$indentation = htmlentities('<w:ind w:left="680" w:firstLine="357" />');
$html = '
<ul class="mylist">
<li>TEXT TEXT TEXT.</li>
<br>
<ul>
<li data-listppr="'.$indentation.'"><span style="text-decoration: underline;">TEXT</span>. Text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text.</li>
<li data-listppr="'.$indentation.'"><span style="text-decoration: underline;">TEXT</span>. Text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text.</li>
</ul>
<li>TEXT TEXT TEXT.</li>
<br>
<ul>
<li data-listppr="'.$indentation.'"><span style="text-decoration: underline;">TEXT</span>. Text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text.</li>
<li data-listppr="'.$indentation.'"><span style="text-decoration: underline;">TEXT</span>. Text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text.</li>
</ul>
</ul>';
$docx->embedHTML($html, array('customListStyles' => true, 'useHTMLExtended' => true));
Please note that MS Word has some limitations when mixing paragraph and numbering indentations such as trying to apply a hanging when setting the first line paragraph style. You can check them trying to create that list with MS Word.
Regards.