Sharing what I learned from a frustrating experience:
I use track changes regularly, when editing my own and commenting on others' documents. Randomly, the changes stopped being visible in my desktop instance of Word. If I opened the web version of Word, I could see the "tracked changes" there, but not on the desktop. I discovered that somehow some of the options and advanced options for my desktop program changed. If you experience the same problem, I suggest you check your options and confirm they match the following:
I had a coworker show me his options and changed mine back to match his. Everything is working as expected now.