Although the cloneWordContent method works correctly, we have checked your template.
The problem is that the block wrappers are using the same symbol than some contents in the element to be cloned ($), and these contents have the same words than the block name (PART INST).
We recommend you to use a unique symbol to set block placeholders:
$docx->setTemplateSymbol('#'); $docx->cloneBlock('PART_INST'); $docx->setTemplateSymbol('$');
or use unique placeholder names for blocks.