Broken Bulk Write API

We are trying to bulk update our client accounts; I think this is the first time we use the Bulk API.

I tried 2 SDK versions and in both got the same error

"INVALID_FIELD_NAME"

invalid mapping. invalid api_name Available_Balance.

despite that this is a valid API field name I double-checked with the docs and our prod settings https://www.zohoapis.com/crm/v2/settings/fields?module=Accounts

"api_name": "Available_Balance",

it is 100% there.

It looks like we cannot update the fields we have to update daily.

Is something missing in the docs? I follow this sample https://www.zoho.com/crm/developer/docs/php-sdk/bulk-write/samples.html