thanks a lot for posting this question here in the forum. In case anyone else has the same problem with their Juno at some point, they might be able to find help here…
It is pretty awesome that you have an oscilloscope because it will make things so much easier.
As you already suspected it has probably something to do with the filter envelope multiplexing. Check the multiplexed envelopes at the output of IC32 on pin 3. You should see all the 6 envelopes here. Make sure they are all there with the right amplitude. They should also not interfere with each other.
If everything is fine here, check IC38, pin 1 (TP7). You should see the same thing here.
Next check IC47, the demultiplexer. measure at the outputs of Opams IC 46, IC44 and IC42. You should be able to see the individual envelopes here. If not IC47 might be broken, or there is dirt on the board making the sample and hold capacitors leaking.