If I understand you correctly, the pedal works fine when the effect is activated. But it looses volume when deactivated. As the buffer is part of the circuit used when the pedal is active, I think that it is functioning properly.
When the foot switch disconnects the circuit, it connects the buffer from switch lugs S2 to S3 then through the jumper to S9 to S8. As the buffer appears to be working properly when engaged, I suspect that the issue may be a faulty foot switch or a bad connection from S3 to S9.
I hope this helps.