PR 12899 modified the best-practice doc to add a way to test whether a boolean field exists by using mutate+add_field to add a static value to [@metadata] and then use mutate+copy to conditinally overwrite. Nice trick! Unfortunately it is broken. Filter decoration (add_field etc) happens after the filter function executes. So the copy is done before the add_field. It has to be split into two mutate filters.