This looks like it is the "full site" displayed on a small screen.
Just scroll to the end. After the question-list, there you will find the usual footer including the option to select "mobile."
It is part of a list with heading "Mathematics" that goes like this:
- Disable Responsiveness
Selecting "mobile" will bring back the layout you have on meta.
At the moment, I am not sure if the reason this changed for you is something that SE did or if you switched it (by accident) earlier. (In the mobile view you can do the same thing and then select "full site.")