As part of a Scrum Master role, one of the responsibilities entail around helping and coaching the Product Owner. It is really critical for the Scrum roles to understand their role and function accordingly. If the product owner is assigning the user stories/tasks, it is the job of a scrum master to make the product owner realize the meaning of self-organization.
In the Scrum Guide, a partial definition of self-organizing is given as Scrum Teams are self-organizing. Self-organizing teams choose how to accomplish their work, rather than being directed by others outside the team.” Self-governance is one of the utmost motivators there is for people doing creative and problem-solving types of work. Assigning tasks to people is an implicit claim that the product owner knows better than the team, this not only passes on the wrong notion but it also lowers the morale of the team and at the same time defies the scrum values and Agile principle – “Point 5. Give them the environment and support they need, and trust them to get the job done.” And “Point 11. The best architectures, requirements, and designs emerge from self-organizing teams.”
Hence, the product owner is assigning work, the Scrum Master should intervene and help the Product Owner in understanding the true sense of working in a Scrum framework and refrain him/her to continue this practice.