Applications can grab key sequences for themselves and not pass them on to the application you think should be getting them. Do you possibly have one of these applications running, intermittently/on demand, and your seemingly random issue is related to this application running/not running, and not to a specific Word document?

You should know if you installed such an app however. e.g., I use one such program for my mouse buttons (not for my keyboard keys, but those exist). My program is "imwheel" (this is on Linux, but Windows equivalents exist). I have imwheel configured, for example, "If the 'xviewer' program has focus and I click the 'thumb1' button on my mouse, send an 'F11' key to 'xviewer'".

These kind of programs could certainly mess with your mind if you didn't know they were installed. But either you would have installed it, or maybe you're using a computer shared with someone else who installed such a thing?