Are you using any XSL file for transformation which does the job of excluding the particular file from including? Wix should harvest all the files present inside a directory , It does not harvest a specific file . Your heat command will help to diagnosie .

Alternatively you can write a File copy command in WiX for the particular file to copy from the source to destination. Similar to below :

<Component Id="cmp1F85" Directory="INSTALLFOLDER" Guid="*">
            <File Id="filC6A7" KeyPath="yes" Source="SourceDir\xyz.json" />
        </Component>