Custom Record Letter Merge (Numbering and Prefix Enabled) does not populate Prefix when using NLID to populate the ID field

User has a custom record where Mail Merge is enabled. User also enabled Numbering and is using a Prefix. User created a Letter Template and used NLID to populate the value of the ID field. However, when document is merged, the ID only shows the Number not the Prefix.

Sample setup below:

Custom Record Setup:

Merged Document using NLID: 

To resolve this, perform the following steps:

1. Navigate to Customization > Lists, Records & Fields > Record Types.

2. Edit the Custom Record.

3. Under Fields tab, click New Field. Use the settings below:

    Label: Provide Label
    Type: Free-Form Text
    Store Value: No
    Validation & Defaulting > Default Value (Formula: Checked): 'ACT' || {recordid}
      *Replace 'ACT' with the prefix for the Custom Record

4. Click Save.

Use the ID of this new field in the Letter Template to populate the ID with Prefix.

Merged Document using NLNAME: