I’m having the same issue. The API docs don’t mention the associations field being supported, and it appears to be ignored. The endpoint in question is POST /crm/v3/objects/contacts/batch/update.
It would be really helpful if there was continuity across update and create, rather than getting involved with the associations API.
Did you get an answer on this? the same seems to be true for upsert, the association fields are just ignored. this is problematic because if i upsert a new contact, hubspot by default will create a company with the domain, which could be not the company’s actual domain. if i don’t let hubspot create a company automatically, if someone were to create a contact in the UI, it will not have a company. love your experience.