Hello,
cloneBlock does inline type replacements by default when using WordFragments (https://www.phpdocx.com/api-documentation/docx-path/clone-blocks-in-docx):
type string Possible values are: inline (default) or block. Used by WordFragment values.
so block contents (such are paragraphs) are removed from the content, and only inline contents are added.
In this case you need to do a block type replacement to keep block and inline contents:
$docx->cloneBlock('SAMPLE', 1, $vars, array('removeBlockPlaceholder' => true, 'type' => 'block'));
We also recommend you create the new custom list style in the DOCX instead of the WordFragment, so it can be reused.
You can read more information about working with inline and block contents on https://www.phpdocx.com/documentation/practical/wordfragments-and-wordml (Tricks and tips section).
Regards.