Hi Celeste,
After a bit of digging around, I found an example in our system. The cause seems to be that the resource's schedule had a non-working day exception (i.e. public holiday). The legacy resource planner is not showing the planned hours for this day, even though there were indeed planned hours for that day. Interestingly, this logic was carried over to the new Calendar tools, but I believe they are going to change it to show planned hours even if the user is on a day off / schedule exception.
Regards, David Cornwell