The Mailchimp currently displays a maximum of 10 merge fields (the last 10 created), which is a problem if you have more than 10 merge fields.
This error was reported in September 2023, and the n8n support staff suggested it may be "it might be a pagination issue within the node " but no fix has been applied.
Hey @David_Hawley , the internal ticket was created back than and is still in backlog due to relatively low popularity of using this integration by n8n users. Simply put, unfortunately the the engineering team hasn’t started working on the fix yet.