Hello,
There's no known issue in phpdocx 7.5 working with HTML images using the supported image formats and options.
Maybe you are not adding the images correctly. Please note that since phpdocx 8.5, the downloadImages option available in embedHTML (https://www.phpdocx.com/api-documentation/word-content/embed-html-Word-document-with-PHP) is true as default, but in previous versions this option is false as default. Maybe you need to enable this option or the images you are adding can't be readed/get by PHP.
On Insert images with HTML you can read more information about adding images with HTML. Please note that some features detailed on this page are only available in the latest versions of phpdocx. In the package you can find samples of all methods for the version you are using.
We recommend you upgrade to the latest version of phpdocx to be able to use all features included since phpdocx 7.5: https://www.phpdocx.com/news/
Regards.