This was driving me crazy. After doing some digging through the MongoDB.EntityFrameworkCore repo I found that the ef providor has its own conventions in MongoDB.EntityFrameworkCore.Metadata.Conventions. You add a convention with the method override on your dbcontext
public class MongoDbContext : DbContext
{
protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
{
configurationBuilder.Conventions.Add(_ => new CamelCaseElementNameConvention());
}
}