When you create a new Task in Planner the default behaviour is to not assign this to a user.
This means that the Task will be visible in your Planner applications when you log in but it will not synchronise into your OpenCRM Task list, because it has yet to be assigned to a specific person and therefore the Microsoft Sync does not know which user to add this Task to.
It's easy to assign a user on the Task within Planner but you have to;
a. remember to do this
b. edit the Task and select the assigned person
How can we automate this to save me these extra steps and also have more confidence that tasks added in Planner will be synchronised onto my OpenCRM Task list?
The answer is to use Microsoft Power Automate to automate this process. You can create a new flow from scratch OR use the prebuilt template available within Power Automate to always add the "assigned to" to be the same as the person creating the task.
Quick Note : when you create the flow from a template most of the information is pre-populated and easy to select from drop-down selectors, which in turn are populated based on your login details. One field that is less easy to find is the 'Plan ID'. This can be found by examining the link to your Plan in a browser - the Plan ID is the part highlighted in this image;