Hi @sparklepimp ,
Here's the [https://www.manualslib.com/manual/688325/Technics-Sa-Ex100.html?page=16#manual|phono section] of the service manual for the receiver, showing the input circuit.
I'm at a bit of a loss to understand why the volume would change when swapping the L & R inputs because if the right phono input in the receiver was faulty then it would always be faulty whether the left output or right output from the turntable was connected to it and not change to the left input.