I have a Word document with one mergeField who has name "NBJ"
I open document.xml and I found
<w:instrText xml:space="preserve"> MERGEFIELD NB</w:instrText>
The document works fine, but I don't understand why I haven't the same merge field name ?
Field coding can be split into more than one element, so if you have another look at your document's XML my guess is that you will find another such element whose text content starts with "J".
In general, when you are dealing with XML encoding in Word, you need to be aware that Word frequently splits texts into several pieces. It certainly isn't always obvious why, but two common reasons are: