Many Tomtom models have that bug. When you do "show home/work traffic" it recalculates the route from your house to work, and erases the destination time.
That would otherwise be fine except that it never shows the time again while driving. And if you keep driving on that route, the time never shows.
If you drive off the route, it recalculates a new route from your new position, and then shows the time.
I've contacted Tomtom about the bug, but the haven't confirmed if/when it might be fixed. As with all Tomtom bugs, call Tomtom about it, the more calls they get the more they prioritize a particular fix.
A workaround would be to Navigate to your work (you can add it as a favorite or favorite destination to make it easier), and then tap the lower right corner. Tapping the lower right corner shows the traffic from your current position to your destination, and doesn't cause the arrival time to disappear.