It is on my IPhone 8, I've the latest version of the IOS and I downloaded the application (planner) on it. I don't think any of the scenarios mentioned below is common on the task I tried to comment, but what is in common is that all of them were previously completed, so they are close. Weird thing is when you go to planner in Team, you open a close task and you can comment anyway without this message, same if you do in your PC, you can open directly planner or go through Team and comment on completed task.
