I wouldn't be surprised if it goes away when a hotel gets "close" (whatever that may mean) to being sold out. I doubt that when there's only one room left there's still Advance Purchase options. And, if so, since it's theoretically possible for a hotel to get close to sold out at any time, it would mean that Advance Purchase could disappear at any time.

And I don't have any idea what "close" could be in this case. It could be something far from close (say, just any particular percentage of rooms already booked) for all I know. I seem to recall in some hotel chain (but don't remember which one) that some discount rate (again, I don't remember which one) was only available before the bookings went above something like 65% capacity.

(This would be analogous to "fare buckets" at airlines, where once a "bucket" of seats at a particular fare sells out, that fare automatically stops being available. But unlike at airlines, there no way to check if it's Y7 H5 Q3 rooms remaining at hotels, AFAIK!)