BroadcastHub doesn't wait for first materialization of Source - akka-stream

Question

This is what Akka Streams documentation states about Source materialized from BroadcastHub Sink (link):
The resulting Source can be materialized any number of times, each materialization effectively attaching a new subscriber. If there are no subscribers attached to this hub then it will not drop any elements but instead backpressure the upstream producer until subscribers arrive.
Therefore I would expect that this:
Source<String, NotUsed> bcast =
Source.from(Arrays.asList("1", "2", "3"))
.log("before broadcasting")
.runWith(BroadcastHub.of(String.class), materializer);
bcast.log("after broadcasting")
.runWith(Sink.ignore(), materializer);
would effectively backpressure the upstream of the BroadcastHub Sink until the bcast Source is materialized at least once, but it is not the case. If I run the above, the output is:
[before broadcasting] Element: 1
[before broadcasting] Element: 2
[before broadcasting] Element: 3
[before broadcasting] Upstream finished.
[after broadcasting] Upstream finished.
which is contrary to the documentation, as the elements are effectively dropped and never broadcasted. If I limit the buffer of the Broadcast Sink like this:
Source<String, NotUsed> bcast =
Source.from(Arrays.asList("1", "2", "3"))
.log("before broadcasting")
.runWith(BroadcastHub.of(String.class, 2), materializer); //buffer smaller than number of events
then all the elements are broadcasted:
[before broadcasting] Element: 1
[before broadcasting] Element: 2
[after broadcasting] Element: 1
[after broadcasting] Element: 2
[before broadcasting] Element: 3
[before broadcasting] Upstream finished.
[after broadcasting] Element: 3
[after broadcasting] Upstream finished.
The way I see it it's either a bug or the documentation is misleading?

Answer 1

I am not an Akka Expert, however, I did some more tests in Scala. (and also changed from strings to integers)
By throttling the stream to make sure the second stream connects before the hubs upstream source has completed, it works as one would expect.
If the delay of 100 millis is changed to 10 millis then the second response (below) is observed.
I havent checked the source but ...
It looks like the elements do get buffered by the broadcast hub, but if the upstream source has completed when the second sink connects, it sees that the upstream finished, and completes, without receiving the buffered elements.
I'm not sure if that's a documentation issue or a bug.
val sink=BroadcastHub.sink[Int]// (2)
//10 millis
val bcast = Source(1 to (3)).throttle(2,100.millis).
log("before broadcasting").
runWith(sink)
//Thread.sleep(1000) doesnt make any difference
bcast.log("after broadcasting").runWith(Sink.ignore)
100 ms throttle Result
[DEBUG] [01/29/2020 10:48:50.168] [broadcastHubTest-akka.actor.default-dispatcher-4] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Element: 1
[DEBUG] [01/29/2020 10:48:50.168] [broadcastHubTest-akka.actor.default-dispatcher-4] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Element: 2
[DEBUG] [01/29/2020 10:48:50.172] [broadcastHubTest-akka.actor.default-dispatcher-2] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [after broadcasting] Element: 1
[DEBUG] [01/29/2020 10:48:50.172] [broadcastHubTest-akka.actor.default-dispatcher-2] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [after broadcasting] Element: 2
[DEBUG] [01/29/2020 10:48:50.233] [broadcastHubTest-akka.actor.default-dispatcher-4] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Element: 3
[DEBUG] [01/29/2020 10:48:50.233] [broadcastHubTest-akka.actor.default-dispatcher-4] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Upstream finished.
[DEBUG] [01/29/2020 10:48:50.233] [broadcastHubTest-akka.actor.default-dispatcher-3] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [after broadcasting] Element: 3
[DEBUG] [01/29/2020 10:48:50.234] [broadcastHubTest-akka.actor.default-dispatcher-3] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [after broadcasting] Upstream finished.
10 ms throttle result
[DEBUG] [01/29/2020 10:56:26.030] [main] [EventStream(akka://broadcastHubTest)] logger log1-Logging$DefaultLogger started
[DEBUG] [01/29/2020 10:56:26.031] [main] [EventStream(akka://broadcastHubTest)] Default Loggers started
[DEBUG] [01/29/2020 10:56:26.136] [broadcastHubTest-akka.actor.default-dispatcher-2] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Element: 1
[DEBUG] [01/29/2020 10:56:26.137] [broadcastHubTest-akka.actor.default-dispatcher-2] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Element: 2
[DEBUG] [01/29/2020 10:56:26.137] [broadcastHubTest-akka.actor.default-dispatcher-2] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Element: 3
[DEBUG] [01/29/2020 10:56:26.137] [broadcastHubTest-akka.actor.default-dispatcher-2] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [before broadcasting] Upstream finished.
[DEBUG] [01/29/2020 10:56:26.139] [broadcastHubTest-akka.actor.default-dispatcher-5] [akka.stream.Log(akka://broadcastHubTest/system/StreamSupervisor-0)] [after broadcasting] Upstream finished.