The single date picker doesn't allow month selection from the dropdownlist in the last year
There is a bug with the single date picker.(even on the demo page)
The month dropdown doesn't allow selection with the last year of the date picker chosen.
Every other year works just fine, and month selection with the dropdown is possible.
When you select the very last year in the date picker though the selection list opens, but selecting a month is not possible.
This is not a bug. You can control this by changing the max year value as shown below: