"Mid-Side" giving lop-sided output

I’m pretty sure I’m asking the laws of physics to break themselves because there’s a reason why “Mid-Side” is in quotes: I’m trying to fool the M-S matrix into thinking it’s reading a Mid-Side audio file, when it is in fact getting left and right mics for two live singer/guitarists, the idea being to put the lead singer in the middle and spread the other (bvs) as the “side” channel.

What I’m getting is a very lop-sided output instead. I’d tried this with another session, where the ‘mid’ was a vocal/guitar mic and the ‘side’ was a miked-up amp, and it worked very well. But not here. I’m showing a matrix on the accompanying pic but it’s the same with a plugin.

I’m trying out this was because I’ve tried the Haas Effect trick with a stereo delay from Dom’s videos; it’s okay but it can lose definition. When it works, this seems cleaner.

Can anyone tell me why I’m getting this lop-sidedness?