So you missed the nuance of my two follow up replies. Asserting that the problem comes from another plugin or theme, simply passes the buck and is unsatisfactory.
In my first follow up, I noted that images do appear for subsequent posts after selecting the “Show More” button. So what is different with in your code in the rendering of these items that keeps the other plugins from interfering? Maybe your implementation can be improved for the initial rendering.
In my second follow up, I noted that after purchasing your pro version, the problem went away. Clearly there is something in the pro code that can be used as a model to modify your free version that avoids the issue of not showing the Featured Images.