I'm using the newest CMC v 3.0 on Joomla 3.6.2 and am running into issues when trying to edit a user's subscription status in the backend component. If I try to change the subscription status, I'm getting the following errors:

COM_CMC_SUBSCRIBE_FAILED 400: is already a list member. Use PUT to insert or update list members.

Based on the error, it seems that the API call is using the wrong HTTP method when the user has already been synced and is not a new addition but an update.