probably Brian means that when it is patched to the stillson, maybe the stillson doesnt go exactly at 0v when on Gate Low , so this might be the reason for the odd behaviour when patched to the Stillson, the grounding is not an issue, the fact it didn’t probably close at perfectly 0v is. (tip: maybe it can be calibrated with trimmers! a lot of modules , example: analog envelope generators, don’t close perfectly at 0v, usually with analog devices this might not be a problem because there are tolerances, but with a very precise machine such as er-301 this might become noticeable)
but when you unpatch from the stillson and leave the cable patched only on one end to the 301, it is normal to have some voltage going on! so this is the reason for this behaviour, and has nothing to do with the other behaviour.
same result, different reasons