Looks like all those Envoy seats were just being held for some reason after all, or some large trip rescheduled. I checked back on that same flight that previously had no seats available in Envoy, and now it has 20 (of 28) of the seats open. Seems like a lot of lost potential revenue if they were just being held.
On the plus side, there's probably a good chance that I use miles toward upgrading that segment now.