Hi Mike,
Here are the answers to your questions:
1.
I asked my colleague and he confirmed, that we have an Exchange Online License.
2.
At least at the moment, my newly created task "Test", shows the comments "Test1" and "Test2" to me. I created the task "Test" in the new bucket "Test". After
that I tried to create a task "Test2" within an old bucket and posted the comments "a", "b" and "c" to it.
Result: No problems by reopening its task card. All posted comments are there.
I moved a task with that problem to the new bucket "Test" and opened its task card.
Result: The same issue occurs and comments will not load.
As I mentioned before, a task with that issue also accepts new comments and also will show them if you reopen its task card.
The behavior for a new plan and new tasks there is the same as the behavior for new tasks in an existing plan:
That problem will not occur. Perhaps it will occur tomorrow – but not by now.
3.
Other users will run into the same issue if they open the task card of a concerned task.
4.
I do not know, how to reproduce the issue for a new task, but I want to provide a screenshot of a concerned task card to you:

Thank you for your assistance on this.
Regards,
Marc