Hi Grant Edmond,
Greetings. Thank you for posting the thread to our community.
I understand your concern, it seems you cannot fully see the task history info of Planner premium plan. For the situation you’ve encountered, I do understand the inconvenience caused and feel very sorry about it.
Due to this situation, to help you further with your situation, I have sent a private message to collect information from you, you can post your reply there so it can protect your privacy and data. please access the private message via this link: Private Messages - Microsoft Community.
Appreciate for your patience and understanding, and Thank you in advanced for your cooperation!
Hope you having a nice day and wish you all the best as well. 😊
Sincerely,
Harry | Microsoft Community Moderator