When using keyboard navigation with the dropdownlist, combobox components, you cannot have no items selected in the popup. When you reach the start or end of the list, hitting the up/down arrow key wont deselect the item. In contrast to this behavior, the autocomplete popup doesn't behave the same way. When you reach the start or end of the list in that component, hitting the up/down arrow key will result in none of the items being selected. This is poor user experience in my opinion.
Imagine typing in the autocomplete input box and the popup shows only one result. By (accidently) hitting the up/down key, the item gets deselected and hitting the enter button after that will result in not having selecting an item from the list. Even more so, once you have closed the popup you cannot open the popup again with keyboard navigation. The popup will only get shown again when the text in the input box is changed. Again, this is poor user experience in my opinion.