In my list, I can disable sorting and reorder tasks manually. Why is this option turned off for employees to whom I assign new tasks? In a situation where everything changes dynamically and many small tasks are added, the tool has now completely lost its usefulness: (((
Hi @Maciej_Bartkowiak, welcome to the Asana Community Forum!
This is related to a recent update, you can learn more about it in New! Introducing a membership system for My Tasks 
If you don’t have access to manage another user My Tasks, you will only be able to filter and reorder. Once the user grants you access to manage their My Tasks, you will be able to manage their My Tasks list.