I am having the exact same issue. I have clients that use the Bill Number for specific purposes and every time you open an invoice and save it the Bill Number gets automatically replaced by MYOB. To my knowledge no-one has asked for MYOB to do this as I have never seen a post for such an action.
This action serves no positive purpose whatsoever but only causes problems for us, the users. This needs to be fixed... yes, I said "fixed" because this is a fault in the system not anything beneficial to the user.
For us, this action means that every time we open an existing invoice we then need to click on 'Save' and then before closing out of the invoice we need to go back to the top of the invoice and revert the Bill Number back to the correct reference and then click 'Save' again before closing the invoice. This is a case of MYOB giving us more work to do.