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Hi Robin,
Thanks for posting the query here in the community.
According to my research, the comments are stored in the conversations. When we delete it, comments will also disappear.
I suggest you try following and see if you could find the comments.
Please check if you can view the comments in the affected group Conversations tab in OWA.
Go to Outlook web app, find the related group of your plan, take a look at all conversations for your task and see if you can see the comments.
I test from my side and find that if the group owner(the plan owner) deletes the group conversations in Outlook(or Outlook on the web), those comments also disappear from the task in Planner.
So I suggest you contact the owner of the plan, check if he can see those comments in Planner and in conversations from Outlook on the web. If not, confirm if he deletes the group conversations.
For your reference: Office 365 Planner Comments Disappearing, Office 365 Planner Comments Disappearing.
If you still cannot find the comments, I'd like to confirm information below for troubleshooting, thanks for your time and effort.
1.Do all colleagues in the same plan have this issue? Whether there’s still any member can see these comments?
2.Does the issue happen in a specific task in the plan? Do other tasks have the issue? Do other plans have the issue?
Best Regards,
Waqas Muhammad