Once a Task is assigned it can be locked so that it doesn’t get transferred to a different route when reoptimizing. It will still be able to shift around within the route.

To do this, right click the Task you wish to lock and select Lock.

At the moment, locking a Task does not mean it will stay in the Route - it means it will only be associated with the Driver whose Route the Task is a part of. However, if there are other, better solutions where other Tasks fit into that Route, it is possible that the Locked Task might get pushed out of the Route in favor of fitting another Task. The only way to prevent Tasks from ever falling out of the Route is giving them higher priority, or locking the Route.