It looks to me like there may be rendering issue here when the x values are datenums as opposed to datetimes. I added a scatter plot to show where the bars should be. You can see the second grouping is missing some bars, too.
However, if I zoom in, the missing bars appear. If I zoom out, they disappear again.
One small additional comment: I don't think the 'keepticks' option in datetick is doing what you think it is doing. I would leave it off. The x axes align better that way (in your orginal figure, Mar appears twice, for example).
title('time as datetime')
One last comment. These figures are rendering differently here that in desktop MATLAB. Be sure to test this on your computer to see what I am trying to explain.