It looks like the changes in PR #326 had this side-effect.
Now that the "allowCreate" user input item isn't in this.state.filteredOptions it doesn't get hit in focusAdjacentOption() during keyboard nav.
The options I can think of here are either having focusAdjacentOption() take allowCreate in to account as a possibility outside of the filteredOptions array -- or -- when filteredOptions is built add it in at that point instead of during buildMenu.
There might be others. I'm willing to hack on this if there's a good path forward.