It seems that Facebook’s Comments Box Plugin display order is “Social Relevance” order, not chronological.

Social Relevance: Comments Box uses social signals to surface the highest quality comments for each user. Comments are ordered to show users the most relevant comments from friends, friends of friends, and the most liked or active discussion threads, while comments marked as spam are hidden from view.

・Official: Comments - Facebook Developers

There are also articles explaining it in Japanese translation.

Social Relevance: Comments Box uses social signals to display the highest quality comments for each user. Comments are arranged in order of what is considered beneficial to the user: those from friends, those from friends of friends, those that are most liked, and those that are most debated. Meanwhile, those marked as spam are hidden.

・Source: Comments : Japanese Translation and Tips for Facebook Developer Documentation

It seems that customizations such as displaying in the latest post order cannot be done.

That’s all from the Gemba.