A project team has a senior subject matter expert (SME) who is comfortable with data...

Explain to the SME the importance of being a team player, that the tasks are assigned by the scrum master, and work should not be refused.

Acknowledge that the SME has a valid point from an efficiency point of view and will perform better staying as a specialist.

Leverage a separate testing team altogether to assist with all testing to ensure consistent results.

Explain to the team the importance of reducing team size by reducing specializations and bottlenecks and that every team member plays an equal part.