I don’t know the plugin you are talking about, but in general you start with a cardiod (mid) and figure 8 (side) signal.
Stereo left is mid and side summed (M+S), stereo right is mid and side summed with the polarity flipped on the figure 8 mic (M - S).
It sounds like you have a stereo signal and are trying to decode the mid and side components? If that’s the case you should be seeing (L+R)/2 for the mid and (L-R)/2 for the side.