When Inserting Merge Fields in Word, and using a template with very many fields, I can’t see some of the merge fields. It looks like the list is incomplete.
Microsoft Word’s Mail Merge Manager has a limit on the number of merge fields that it can show. It stops listing all the merge fields after that limit is reached.
The limit may vary with different versions of Microsoft Word.
Split your merge_fields CSV file into 2 or more parts, such that each part does not exceed the limit. You can then work with each part (separately) in the Mail Merge Manager.
To split the file into 2 parts, you could make 2 copies of it and then use a text editor (like WordPad or Notepad) to remove the 1st half of one and the 2nd half of the other. Avoid editing the CSV file in Microsoft Excel.