You both missed a detail; you must be recording to a MONO track type.

Check the MONO input selection ROG recommended in his intial post.
Go the track you want to record on and right-click by the track name, and select 'mono audio' as the track type.

Hit record and keep take.
RB will record the left channel input on the selected track and put the right channel on the next available (blank) track.


I do not work here, but the benefits are still awesome
Make your sound your own!