Yamaha CS80

There is nothing between the SH and M but a wire. Output K1 of SH board is bound to the K input of M11 and an other wire goes from there to the K input of M21. So most certainly, the issue does not follow this path. Try and check the V input of the M boards, the one where the vibrato arrives.

Does the pitch variation really concern all boards (M21 to M28)? If so, could it be a damaged "Detune ch2" slider?

Joachim