The behavior you are seeing is exactly what should happen, @danny. You can never create a new sales item in the item box in a sales invoice. If you thought that is what happened, you misinterpreted what Manager did. As you type in that box, Manager brings up the list of pre-defined sales items containing the string of text you have typed so far. As you type more, the list narrows, often quite rapidly, until there is only one item left. This can make it seem like you are entering text, but you are not. You are only entering a search string.
On the other hand, if you only click on the dropdown arrow, you will get a list of some or all of the pre-defined items to choose from. I don’t actually know what the limit is on the number of items, since I only use a few. I suspect that if you have more sales invoice items than the dropdown will display, you probably get the most recently used.
The other thing that you might have done was enter the invoice item in the Description box, which will accept any text you place there.