By 'the bottom-most line', do you mean the set of code that's currently been typed into the console, or do you mean the last actual line in the console history (ie, code that's already been submitted and run in the R console)?
If you are talking about the currently typed code in the console, I think I'm seeing the same behavior as you -- double-clicking a word seems to only occasionally highlight it, while triple-clicking does always highlight the whole line. Does that match your experience?
Let me know if this is the behavior you're referring to, and we can try to further investigate!