My cheshbon:
I booked 10 ow SDR-MAD, typically one per week in the spring time.
Did this same pattern for each of 5 individuals: myself and DW (old accounts) each of our 3 children (new accounts).
BH, DW and I had our points posted and they remain. In fact, I had a previously booked award flight that would conflict with one of these SDR-MAD flights, but BH that didn’t seem to bother anyone.
Our three children’s accounts got bupkus. One got the request for ID. One I can’t remember. One got “you opened new account and had an old account, so we canceled the new one, and BTW you won’t get Avios.” That was my oops — didn’t remember that DD had a prior account and had her make a new one.
I’m waiting to get past the 330 days to book for me and DW (and hoping there is no clawback before that day arrives). I suspect that none of the kids will get their Avios, so I will try to get their flights refunded.