Wayfinder and URL issues for parent documents

I have put together a 2-level Wayfinder menu that was working fine at first, but then it suddenly started acting up.

Whenever I would click on a menu option from inside a parent document, it would append the parent directory in front of the target link and result in an error.

Each time I clicked the parent link, it would append again. For example, after clicking the link three times, the url in my browser would show:

mydomain.org/parent_dir/parent_dir/parent_dir

Then when I click another page in the menu, I'd get this:

mydomain.org/parent_dir/parent_dir/parent_dir/other_page.html

I fixed this by adding &fullLink=`TRUE` to my Wayfinder declaration, but I still wonder what started the problem in the first place.

(I also have made all the minimum settings for friendly URLs, which seems to be working.)

Any ideas?