Thanks for reaching out. Are you referring to BuyXGetY discount codes? Taking a look at some other posts here in the Shopify APIs and SDKs board, it appears as though this is expected behaviour and your best bet is to use the GraphQL API instead of the REST API, as these discount types are fully implemented in GraphQL.
I managed to find two forum posts that offer further explanation, which you can find here and here If you have any followup questions, feel free to respond to either of those posts directly.