This is a problem of no consequence at all….. In any M/S system there are tiny errors in levels between L and R due to small differences in the actual values of resistors and particularly in the accuracy of pots. If your unit has a ‘width’ control, then altering the width will allow you to correct for any error in the M/S matrix. Some of the early compressors did not have a width control and the ‘difference’ channel was adjusted to give either exact L/R or sometimes a slight bias towards extending the width of the image…. which is exactly what you describe.

If you demand a total lack of ‘crosstalk’, then frankly you have got the wrong equipment. All I can advise is to listen to the result of what you have, if it’s not what you need then sell the equipment and buy something that is scientifically more accurate…. but will it sound any good?

I’m sorry to be astringent, but I don’t see a problem here. 🙄