on_data_available provides a reader that is None

I'm attempting to do dynamic topic discovery through the BuiltinTopicReader. When I set my configuration file to be multicast and between two devices on a subnet everything works as expected. When I try to do the same thing using "lo" and no multicast instead the listener gets None instead of a data reader.

I'm not sure if I'm just going about this wrong completely or if this is an actual bug...

class TopicListener(Listener):
    """Receive topic information from the builtin reader."""

    def __init__(self) -> None:
        """Init."""
        Listener.__init__(self)

    def on_data_available(self, reader: BuiltinDataReader) -> None:
        """Read topic information and store it in the mapping."""
        if not reader:
          print("Failed to get reader")

I've played around with the tracing and haven't found anything that seemed helpful. I.e. info/warning/severe were completely empty.

Reproduce the BuiltinTopicReader listener behavior with multicast between devices, then with "lo" and no multicast, and compare whether on_data_available receives a reader. Start by inspecting the listener callback and the configuration used in the report; review the empty tracing output as needed. Done means the loopback configuration either supplies a valid reader for dynamic topic discovery or clearly documents the expected limitation.