A couple more things which may aid in diagnosing the issue:
1. When, for example, a single word has been selected, and Ctrl-b (or "B" on the ribbon) is pressed, the selection expands to the entire document. This selection persists even after either pressing Ctrl-b a second time or Ctrl-z.
2. If I select a word, go into the Font dialog box, and select Bold, just that word changes state.
Hoping to hear from someone with more things to try.