This is likely because the Business Group entity exists both in the vRA Appliance and the IaaS SQL DB. In 6.2 we can modify the BG directly through the API (I recently wrote about this here - though the lack of IaaS means this isn't actually a supported config). Interestingly, when I perform a GET against a Business Group, I don't see the custom properties in the response. Very weird.
I'll need to dig a bit further on this.
Grant
