Usually when search results pull a page’s content into a section for an excerpt, it runs the content through a filter which runs strip_tags or some equivalent WordPress specific function that removes any and all HTML tags so as to not break the site due to a opening <div> tag not having a closing partner because the excerpt cut off the ending before it was properly closed, and would then potentially break the layout of the site, and other reasons, but that’s the simplest example.
To customize the behavior, you will probably have to edit search.php or content-search.php (if it exists) to pull in your own version of the content to show, whether it be all the content, or a modified version of the excerpt system.