I was having this issue too and it was driving me nuts. Turns out that, somehow, I turned off “Automatic outlining” which meant that I did see the plus/minus signs and the shortcut Ctrl + M + M didn’t work either.
After I turned on Automatic Outlining in Edit > Outlining > Start Automatic Outlining everything went back to normal.
NOTE: On some ocassions, for this solution to work, you also need to close and reopen the document before clicking on Start Automatic Outlining. In fact, sometimes that option doesn’t appear in the menu if you don’t do so.