If you look at [[todo/org mode]], the link to the Discussion page is not there (has a question mark), as if it didn't exist. But--through the search--I discovered that the Discussion page does exist actually: [[todo/org mode/Discussion]].
So, there is a bug that prevents a link to the existing Discussion page from appearing in the correct way on the corresponding main page. --Ivan Z.
+
+Perhaps, this has something to do with the same piece of code/logic (concerning case-sensitivity) as the fixed [[bugs/unwanted discussion links on discussion pages]]? --Ivan Z.