I was incorrect in stating that you can only zoom in to show weeks. I forgot there was also a day setting. Either way, tasks that only have a due date (not start date) don’t show a block of time like the rest. BTW because there aren’t currently start times in Asana, a task that has matching start and due dates will update to only show a due date.
That said, the task appears the way you would prefer when you hover over it from within the timeline. Did you notice that? Given such, you might have a good case for a visual change. I changed this post to be within the #productfeedback category so you and others can upvote this recommendation.
I’m sure timeline is a much better view for your workflow. I only mentioned Calendar because from that view you will see tasks have the same visualization regardless of whether there is a start date or not.