Hi @Virginie6 , it sounds like this amy be the root of the problem that you are naturally trying to avoid.
However, people having tasks assigned to them that are months away isn’t necessarily a bad thing. It has it’s advantages such as populating workload graphs and feeding dashboards.
You may want to explore ways with the Engagement Manager of how they manage their My tasks and how they could hide the tasks that are months out by using filters and exploring the use of various saved view tabs.
Here is a great article by @lpb
Just my 2 cents.