A Microsoft application for managing email, calendars, contacts, and tasks across devices and platforms.
Looks like my issue was resolved when I was able to update the device to El Capitan last night. Specifically installed 10.11.3, and now the column headers are displaying again. For what it's worth, I haven't installed any patches to Office since I reported this, so perhaps is unique to Yosemite. Either way, it's fixed for me know.