Amex allows users to create additional cards for their friends and family. These are called supplementary or additional cards. The transactions made on these cards show up under the main card's transaction and the balance on them is the same as the balance on the main card.
When consenting on AMEX's website, these cards will be shown, however when the
/cards endpoint is called, TrueLayer will only return main cards. All supplementary cards will be removed from this output.
The reason is that there is no additional information on these cards that is not present on the main card.
Read more about supplementary cards here.