Yup, it appears to be intrinsic to the voice; I'm sure it's set to 'auto release' with each note with no sustain added because the decay is already quite long, and if it behave like legacy pads and strings (which hold indefinitely with the sustain pedal). Still, if what you want *is* to hold the sound, it's odd that they didn't really include anything other than left hold that accomplishes that.
I even tried changing the release to 63 (max) and it barely has any effect; they programmed this voice with this specific ADSR in mind,
Mark