I have had this problem when checking in my 5 family members (on one PNR) online. You can get around this by splitting up your check-in. Just check-in 2 or 3 or 4 passengers first, select their seats, then check-in the rest. You can check if there are 5 seats that you like when you get access to the seat map, and if you don't like what you see, cancel the check-in and keep the seats you already have.

Row 12 is not part of the elite prebook seating area. Row 12 was blocked on the 139/320 a/c to allow for disabled and family seating with bassinets. Then AC went and reconfigured the planes and brought in the curtain divideres. However the original block on row 12 seat selection remained.

On the 762 and 763 a/c, row 12 had bassinet position and was not considered part of elite prebook section. However anyone could book these seats. Note the middle seats on row 12 were part of Elite prebook as only th 12AC and 12GK had bassinet postions.

One the E75 and E90, 12A 12D and 12F are elite prebook but 12C is not.

To get your family prebooked into row 12 munerous rimes you must have been flying the 763 transcon. Either that or this was a 23:59 OLCI and moved everyone into the now available row 12.

Admittedly most airlines use the approach to run an overnight job and sweep pax out of the elite prebook sections. UA does this for E+ and others do it for their elite prebook sections. From an IT perspective, the airlines built a simple overnight batch job to run and audit the prebook process. This is the 50 cent solution.

Also, it is possible that the OP booked the seats and then got reallocated the family to row 31 after the night job had run. Did ther OP ever check the seat allocations between the time of booking and OLCI?

I also submit that the reallocation at least resulted in the family being given seats together, this will commute the possible compensation. IT not as if they were scattered throughout the cabin in middle seats.

I did check the seat assignment a couple of days before OLCI and we still had row 14. I even went into the booking engine after OLCI kicked me out for trying to change 5 seats. Guess what - the seats were still showing as row 14.