To replicate:

1. focus on the textbox
2. Type "a"
3. Press "up arrow"
   a. Notice that aria-activedescendent is not set on the textbox although the first entry is highlighted and autocompleted in the textbox. It looks as if visual focus is on the textbox still as well

Note that "down arrow" works if you use it instead of up arrow in step 3 above.

This bug was caught by writing tests! :)