After reading the manual of the EO-102 I realised that by moving the level knob you can “fix” the A input issue. It’s expecting a trigger and its response is expecting a rising edge. not a complete fix as you keep needing to adjust the level knob but it makes it work much better (allows for the tracking of two envelopes which it didn’t before).
By default the first input channel (Channel A) is used as a trigger to synchronise the scope display. An external trigger input is also provided and this signal (which is not displayed on the screen) will be used to synchronise the display if anything is connected to it.
The trigger operates on a rising edge, and the trigger point can be adjusted using the LEVEL knob.