I have tested the navigation when grid batch editing is enabled in the following online example and everything is working as expected:
As you can see when you press Tab the focus goes to next cell with DropDownList and after you click down arrow key the next item in the dropdown is selected. I am not sure what differs on your end but you can compare your setup with the one in the example in order to understand why the behavior in your case is not the same. The online demos are using 2016.2.607 version of the controls.
As a side note, please avoid opening multiple threads with the same questions.
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