It can happen if the theme does some extra filtering in the place where the excerpt is shown.
Our plugin takes over the WordPress function for excerpt generation and sends that output, but if the theme then removes HTML from that output there’s nothing we can do.
First of all apologies for the late response. I use the plugin The Grid (from Codecanyon) to place posts with featured images in a certain layout on a page. In the section below the image where information can be included, there is a section available for the summary (the excerpt) of the post. HTML code that I include in there is not executed (except for the <br> element)