I finally happened to be paying close attention while typing away, during a QSO, and saw what I had just typed go into the CALLSIGN field in the log bar.
I verified this by pressing the TAB key while not being connected to another station, and the callsign in the (not saved) log bar had a blue background after pressing that key, showing it was selected.
Sure enough, what I typed went into that field.
I know there must be another pesky key that is putting the cursor over where the "titles" for the canned messages are displayed, as I've had keyboard input show up there too.
If I stumble across that key, I'll let you know.