I purchased tickets for my brothers family using last years 1-2-3 and miles about 3 weeks before travel. At time of ticketing I got all three sitting together.
On the outbound leg, all the seats changes and the three of them were not sitting together. They had to ask people to changes seats (1 small child with them). On the return, it happened again. At least I caught it on the return and was able to change their seats on-line and get them back together.