It looks like this is still an issue almost 2 years later.
Is there going to be any action on this?
Do I have to continue using the legacy API just for this?
Thanks for bringing this to my attention. I have logged this as a bug and will get back to you with an ETA once our engineers have assessed the necessary fix.
We currently have an engineer doing research on potential solutions for this. When I have more information or an ETA available I will make sure to update the thread again.
The reason this field was returning all 0s is because it is deprecated and no longer actually being used within the DB. This is due to products now being able to belong to more than 1 category (a change made years ago).
The current immediate work around for this is to use the XmlRpc API and reference the ProductCategoryAssign table to find what categories a product belongs to.
Rather than make a breaking change to the v1 API and adjust the field type, or add a new field (adding to the confusion) I am going to be working alongside our e-com team to produce the Rest V2 product API endpoints. This work is still in its planning/design stages, so I do not have an ETA for release of the endpoints yet, but will make sure to update this thread once it is available.