Hi LDC Aust,
I would check if the service invoice lines are dated in chronological order or not.
QuickBooks Online prioritises the service date over # field when sorting line items in an invoice. For example, if there is a later line with an earlier date than the lines before it, that line will display first when printing the invoice. However, if the service dates are the same, the line order should stay the same as the way you have entered it.
Currently the service date sorting cannot be disabled - however if you would like to see an option to disable this in the future I would recommend giving feedback within QuickBooks Online so our Product Teams can further improve the product for you. Here is how you can do it: https://quickbooks.intuit.com/community/Other-questions/How-do-I-submit-feedback/m-p/263002/highligh...