Same thing has happened to us. On an upcoming CW trip we were allocated 2 UD seats which I have since changed to 2 better UD seats!

The trip was booked through BA Holidays & we are only Blue members, so no status either. The only thing that I can think of, is that I did actually phone BA to request UD seats, but obviously that's not guaranteed, & i've really no idea if them making a note on the system makes any difference anyway tbh.

Certainly when I checked MMB a few days later I realised the change/view seats icon was there & went in & discovered our pre-assigned UD seats (60 J/K), which as I say I have changed. Not sure if it's a glitch or the CSA at BA Holidays was doing a good deed, but is very welcome all the same! By the way, holiday was booked 2-3 months ago approx.