Had the same problem once when using an Amex 2-4-1 voucher.
School boy error on my part

The booking had my BAEC number so knew I was gold. When I entered the second passenger, I left out their BAEC number on the basis that this was a reward flights so Avios and TPs was required. Totally forgot that this would mean BA would not then know that the other passenger was also BA Gold.

Took a phone call to sort this out when the missing BAEC number was added. In fact, I worked out the issue (and solution) in the head the moment the agent said that the other passenger did not have BA Gold status. Urrgh!