It’s not a bug. When the “Show Full Post” button is clicked, Discourse attempts to find the post content from its URL. You can tell Discourse what content is the main post content by configuring the Discourse embed whitelist selector site setting.
Details about the setting are here: How to configure the embed whitelist selector setting. Since you have a WordPress site, there is a good chance that the example setting in the topic’s “Finding the CSS selectors to target” section will work for you. When testing this, remember that the post data that’s pulled from WordPress is cached for 10 minutes, so you may have to wait up to 10 minutes to see if your setting is correctly configured.