Between September 13th and September 19th, there was an issue where supporter names were not syncing to Mailchimp. Only emails were being synced. This issue was resolved on September 19th.
You can use the 'Sync Existing Supporters' option on your Mailchimp integration page, to resync any supporters that may have synced during the affected period to Mailchimp to ensure that their names are also included.
You can find out more about the 'Sync Existing Supporters' option and Mailchimp integration here:
https://help.givecloud.com/en/articles/6960156-mailchimp