This happened to me when I took the first BA flight post pandemic in July, albeit in ET. I was travelling alone with my three kids, 12, 7 and 5.. they gave me 2 seats together and split the other two on different rows. When I complained to customer service I was told that I can and should have called earlier and request seats to be assigned together (e.g. ABC and D or C and DEF). Strangely on my next two bookings (one in ET and one in CE, my expectation has been met of having the family of 4 /5
(once my wife was with us) to be assigned on one row, so they may have now put something onto the system to that effect.. BTW I know airlines may consider a kid of 12 a young adult, but in reality he/she is still really just a kid.. I would think they should be at least 15/16 to be considered really your adult and fine to be seating alone.