Hi Marcelo,
I did confirm what you are saying from my own test account. I am going to look into this further and I will post here when i have any updates. As a workaround while this is being looked at, I found that if you create a new Customer with a Sales Rep first, sync this record to Quickbooks, you can then delete the Sales Rep from by blanking out this drop-down field from the Edit Customer screen in Method and pressing save.
-Ben