If the main outline is at it’s smallest width (either with or without the sidebar/inspector showing), it isn’t possible to tab to the due date. I can tab from the action to the context and then to the defer date, but not the due date. If I make the window slightly wider, it’s possible to tab to the due date. From my tests, it appears the main outline (essentially the entire window) needs to be 5 pixels wider than its minimum allowed size in order to tab to the due date.
This issue was reported previously, but it appears that it hasn’t been revisited since 2014.