Can you please tell more what is happening? Does it remove merge fields completely or just formatting switches? Why those codes are “ours”? Did you insert them using DocumentBuilder or what?
If you are talking about formatting switches stripped and you inserted those merge fields using DocumentBuilder, then probably the switches are in wrong positions. It would help if you insert one field in MS Word with all the switches and see where in the field they are positioned and then use DocumentBuilder to insert the switches in the same order.