You might be recording your mono audio through a connection assigned as a stereo bus. Recording in this manner Cubase records only to the left side (you might have your earphones on reversed). You correct this in the VST Connections input menu by creating a mono bus for the connection you are using for your mono source (like a mono mic or guitar). Then make sure this mono bus is assigned as the input to your mono or stereo audio track (it will record a mono file on either type of track).
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