I'm one of the users that actively changes my feed to "Most Recent" any time it isn't. I've noticed that as long as I'm logging in from the same device (typically my desktop/laptop in desktop mode) that it tends to stay the same. I'm not sure exactly what triggers it to switch back to "Top Stories" but I've definitely had it happen when I log in on my Android phone through a browser (typically Chrome) after having to request the desktop version (I dislike the mobile version and uninstalled the app long ago due to battery consumption). Also, it does give the message "Viewing most recent stories · Back to top stories" if it is still on "Most Recent", so the lack of that message is one way to know to switch back.

That's not even what is frustrating for me. I'm pretty sure that it didn't used to work this way, but now whenever somebody likes or comments on a post it is moved to the top of "Most Recent". Just as a poster above has noted, the main reason Facebook wants you to use "Top Stories" is to increase usage, and I would guess that is exactly the reason they are doing this. When I log in I just want to see all the new posts, but now I have to scroll through a bunch that I have already seen just because more people have liked them, and I have to keep scrolling past several posts I've already seen unless I can remember how many likes/comments they had the last time I checked to make sure I've seen everything that's new. Ironically, I submitted feedback to Facebook about this last night.