When an article doesn't have an associated image, the image from the series it belongs to is used. This may be confusing, example https://en.wikipedia.org/wiki/Jim_Crow_laws
Image should only be taken from article content directly.
In above example, series image (which is from South Africa) is used for article (which is about the United States).
This is not an issue with the app per se, but rather an issue with the API that delivers the image(s) to be shown. You'll notice that the same issue happens in mobile web (i.e. go to https://en.m.wikipedia.org and type the search term "Jim Crow laws" -- the thumbnail will be the same).
That particular article includes the {{Segregation}} template at the top, which makes the article "part of a series on Segregation", and that template is where the Apartheid sign is included. This causes the Apartheid image to be the "first" image that appears in the article, which causes the API to serve it up as the "thumbnail" for this article.
The question is, is it possible to limit it to select an image from the article directly, and not from those added to it by templates (such as a series).
No, the algorithm is not that smart and there are legitimate reasons for including a template that has an appropriate image.
It's a bit surprising to me to see a navbox at the top of the page where an infobox would usually be. Is there any reason that is not at the bottom of the page like in other articles?
This task and T91683 are mentioned at https://meta.wikimedia.org/wiki/Community_Wishlist_Survey_2021/Editing/Allow_editors_to_control_PageImage