If the hardware (possibly a software issue, but I don’t think so) was correctly designed it would/should have summed stereo inputs, but SS missed the boat somewhere. This is something I came across sometime ago. There’s also a drumset I created in this post which has instruments hard panned right and left and it does not sum correctly to mono either.
You can try this by plugging two 1/4" TS patch cords into the L(Mono) and R input jacks with the L(Mono) output going to an amp and touch the tip of the end of each input cord. If it sums correctly you’ll have hum from each cable left and right.