I have found some more information about this issue. We had the same problem for other projects and I found out what causes the bug.
The team planner view cannot display task assignments that have a different start or end date than the task. The assignment can get different start or end dates by specifying a custom distribution
of hours for a resource in the Task Usage view:
Solution
Make sure that the start and finish dates of the assignments are exactly equal to the start and finish dates of the task.
This can be done by going to the Task Usage view and entering 0 hours for the assignment in the days that no work will be done.
MS Project will automatically remove the 0 hours from the last day with work to the finish date of the task. To prevent MS Project from removing the hours and automatically update the finish date of the assignment, add a small amount of hours to the last day
of the task assignment (0,02 hrs is the smallest possible amount).
The same process can be used to fix the start date of the assignment.
This is clearly a bug in MPP and I will encourage the client to raise this issue with MS.
