If your application uses the Newtonsoft serializer, you need to install the Swashbuckle.AspNetCore.Newtonsoft package and explicitly opt-in via AddSwaggerGenNewtonsoftSupport().
Swashbuckle.AspNetCore.Newtonsoft
AddSwaggerGenNewtonsoftSupport()
See Serializer Support, and System.Text.Json (STJ) vs Newtonsoft