I phoned the Elite desk again to follow up on my mysterious "missing" seat assignment. The Elite rep passed me on to a supervisor who stated that, as I was told last time, my booking history shows a seat assignment, but the fact that it didn't "stick" appears to be some kind of 'glitch'. Further, he said all assignments are now "restricted" or "frozen" which is something that happens when there is consideration of changing out the metal. Given the segment (IAH -> JAX) and the fact that the plane looks full (and first is not overbooked according to him), the supervisor thought it was very unlikely, though not impossible, that they'd change the aircraft. When I asked what he anticipated would happen when I attempted to check-in and print my boarding passes, he said it might show my earlier assignment or possibly even print out blank in which case the gate agent could pull up the booking history to verify my seat. Ugh. Quite odd. I also queried him if this could result in the system failing to record my mileage accrual, but he felt pretty confident that shouldn't happen. Ah, well, guess we'll see how it goes.