As indicated here by Google:
Our systems analyze the link structure of your site to find shortcuts
that will save users time and allow them to quickly find the
information they're looking for.
Make sure you're not using that URL with a slug in your site, sitemap.xml, or one is linking back to your site, and that there isn't a URL rewrite rule in your web server doing this either. Also be sure to provide the full URL in your sitemap.xml with a high priority value specified for it.
You can add a canonical URL to the page to let Google know the preferred URL is the full one, and then submit the full URL using the Fetch as Google tool to request Google crawl and index it again.
Lastly, you can demote links as covered here, however that would likely not be beneficial to you.