I don't really know if I'm the only one who is experiencing this problem or you as well going through the same.
Well, recently I have noticed that when I log in to Facebook (happened only on mobile devices) and click any of the news feed comments, then the comments are shown at the bottom of the page with the last post.
That's kind of very weird when it doesn't match with the post above them and make you think crazy. :(
After finding the solution I thought why not share with you so that it might make life much easier. :)
It's simple, you can either set the browser as Desktop version or click on the comments and hold it for a second then click Open in new tab.
I've noticed this problem with Android tablet and Samsung S5 mini.
That's all, I hope it helps you.