Update InvcHead.SalesRepList using Updatable BAQ - Not working


I am having trouble updating the sales rep list field on an invoice. Every time I update it, it goes back to what it was before. I can see in the change log that it is changing it to what I would like, but then I see another entry in the change log which is changing it back to what it was.

Can anyone tell me what is going wrong or how I might be able to get this to work?