According to FTM support, not a bug, surprisingly. The software isn't intended to display all relationships according to an answer from them:

"Thank you for contacting Ancestry.com regarding the Family Tree Maker program. The Family Tree Maker program is designed to show a single way you're related to a given person, rather than showing all the ways. That's because our relationship calculator goes through the bloodline; it calculates the closest way you are related by blood to a given person."

This is very disappointing. Other programs, like Legacy, do a much better job trying to display ALL relationships it can find. Didn't know this when I purchased it.