Hello, and welcome.
It should indeed behave as you first describe. It does so on my REV2, resulting in a full ON sustained, full OFF gated sound.
You can perhaps try a few things:
Make sure you are running the latest firmware revision (1.1.0.3 as of this writing).
Perform a full calibration by calling the calibration routine in the Globals menu.
If that doesn't fix it, perform a Globals Reset, to return all the synth to factory default.
Also make sure that no MIDI devices are connected to the synth when testing the envelope behavior, to eliminate a possible external source of the problem.
Hope this helps.