I tested this on my Behringer 2600 and got similar behaviour, so I suspect it's a quirk if the hardware, (one that is possibly not emulated by the Arturia VST). One way around it seems to be to patch a gate into the ADSR gate input instead of using the manual trigger button - try patching the internal sample and hold clock into the ADSR gate input, and the envelope stays flat until you raise some of the sliders.
I wonder if it happens when you trigger via MIDI ? (I didn't test) Patching the KBD gate or trigger from the left hand side into the ADSR would be an option to try too.