It always means I panic (if I had a seat I wanted for some reason) and almost always means nothing. Whenever I ask a GA, they tell me "no, I still show you in 3X" and they are always right.

If you are upgraded, check the upgrade tab on the app - it typically continues to show your assigned seat even when the blank appears in the reservation. Occasionally, what you are seeing is your upgrade clearing.... Rarely, an equipment swap is afoot - check this once at least. Once recently, it meant I was being downgraded.....a flight arrived in ORD simply missing a seat. let us know.