We were able to observe the same issue as reported by you when updating contact using IGmailClient.UpdateContact. The issue only came up when updating contacts for which the required fields aren’t set already. Currently, we cannot point you to any documentation since it needs to be investigated further for the exact root cause. Ideally, the behavior of the API should be transparent for you.
We have logged the issue for further investigation by our product team as EMAILNET-38628. We’ll update you once there is any further information or a fix version available in this regard.