This sounds like a regression from bug 539356... When I start entering some text in a form field, and press the down arrow to try to select an entry, the active style isn't being drawn sometimes. Pressing enter does fill in the field correctly, so this seems like more of a drawing issue than a problem with the autocomplete widget.
Do you remember what page you saw this on? Trying to find testcases for these bugs.
Need more information to reproduce. Please reopen if you're still seeing this with the DLBI relanding.