Mid-side Recording in Cubase

Hi, I’ve been trying to do mid-side recording of an acoustic guitar. As per the widespread advice for what seems to be the simplest way of doing this without mid-side plugins (using a Vanguard Audiolabs V44S stereo mike) I have a cardioid capsule pointing directly at the guitar for the mid and then a figure of 8 capsule at 90 degrees for the side both recording onto mono tracks. I duplicate the side recording, invert the phase and then pan the two sides hard left and right, leaving the mid in the centre. So far so good! However if I increase the sides above a very low level, the stereo out signal becomes heavily un-balanced to the right channel (which strangely is the one with the inverted phase). Obviously I can fix this by panning the stereo out to the left but that doesn’t seem too satisfactory! Be grateful for any advice - am I missing something or doing something wrong? Thanks!