When you set up Copilot for customer service, and you use the new Preview setting "Use Knowledge Sources configured in Copilot Studio", you can then go to copilot studio and add your Knowledge Article entity plus a number of other sources.

Once you do this, "Write an Email" feature is impacted. It no longer can use the Knowledge Articles for answers, only the trusted sources, and it will not present "Resolve the customer's issue" as one of the options.

Write an Email should still be able to use the Knowledge Articles even if you have added additional knowledge sources for Ask A Question.