ableton live – Sum of 2 balanced signals is imbalanced when doing mid-side recording

I’m getting an unexpected result when attempting mid-side recording: The combined signal is biased towards the left side, even though the mid channel is panned center and the side channels are panned left and right evenly. The right side channel is a duplicate of the left side channel with the phase inverted.

What’s the proper way to get this recording balanced?

My mid-channel microphone is an SM58. My side-channel microphone is a WA-87 in the figure-8 configuration.

I’m not sure mathematically what is going on here, since the sum of two L/R balanced signals I would expect to be balanced, but that’s clearly not what’s happening. Adding a delay to the right channel seems to help a little, but has not been adequate to eliminate the imbalance.

phase inversion plugin applied to the right side of the side channel