We are fixing some date-related issues (the due soon/overdue/etc status doesn't always update when it should) in an update coming soon, but I haven't seen anything that would cause the date change itself to be lost.
May help to send some screenshots illustrating the problem to our
support ninjas. To take a screenshot, press the power and home buttons at the same time - you can then email the picture to yourself or us from the Photos app. Thanks for the help!