Why do my created contacts have a last name of Not Provided?

When creating Leads or Contacts, we have to include data in any required fields, just like you would if making them by hand. If you haven't mapped a Mailchimp merge field to the required field or the incoming Mailchimp Member doesn't have any data in that merge field, we will set the value on any required fields to [Not Provided]. 

This avoids the record creation from failing during the sync. To fix:

• Check the correct Mailchimp merge field is mapped to the required field for the object you are creating (i.e. Last Name on contact or Company on lead). You can also use Default Mapping to set the same value on all records. 
• Make sure the merge field is populated on the Mailchimp member. 
• If you have to update any mappings, you can use the retrospective sync or a full refresh to refresh data from Mailchimp to Salesforce. 

Note that other field or record validation errors, plus errors from other custom automation or outages will appear in the Maintenance Logs.