https://reactivex.io/RxJava/3.x/javadoc/io/reactivex/rxjava3/core/Observable.html#concatMapMaybe-io.reactivex.rxjava3.functions.Function-

The diagram says concatMap instead of concatMapMaybe and also shows two items being emitted by the mapping function which is impossible in a Maybe. I guess the diagram was copied from concatMap. :)