It is not because of the Service Item but due to the fact that this product has several UOMs.
For such a case you have to fill in Order Quantity field and Invoiced Quantity will be updated based on the UOM Conversion. That is the way it works in 2.50 also but there the layout is different and this field (and not Invoice Quantity) has a focus once the product with secondary UOM is selected.
In Openbravo 3 this functionality is hidden by default but if Simplification Configuration Script is disabled (like in your case because it was an Upgrade from 2.50) new layout does not handle it properly and proper focus does not work.
We will be addressing it.
In short term you can check if you really need second UOM for this products (if not - field is not displayed and no issue with it).
If you need it - move it (and its UOM) before Invoice Quantity (you might want then to put Start a new Row for this field to Y to have better layout) and UOM so that if displayed it is the next field after the product.