I've noticed that there seems to be an unusually large number of assignments at this building. Currently there are 3 units (605, 1005, 1503) listed on MLS. Since January, there have been three other assignments (903, 1505, 1506). I seem to recall seeing a handful throughout 2017 too. I don't really keep track of this sort of thing, but this seems much higher than I would have expected and what i've seen with other buildings.
Maybe some buyers were surprised by 989. I just think with occupancy so close, people would be better off taking possession and then selling. Seems like you'd get more money that way and not be subject to some of the restrictive aspects of selling via assignment.