Thanks for looking at the logs.  I don't think I'm explicitly enabling frequency agility or bg scanning channel (I've attached the code I use to open the channel below) and I do remember trying to get the RSSI out just to realize that I couldn't, but thanks.  I'll look into cleaning up the channel init.
Channel init code:
ANT_Channel workerchn;
ANT_Device workerant; ...
                workerchn = workerant.getChannel(0);
                if (
                        !workerant.setNetworkKey(0, ANT_PLUS_NETWORK_KEY, ANT_PLUS_TIMEOUT) || // Set ANT+ network key
                        !workerant.enableRxExtendedMessages(true, ANT_PLUS_TIMEOUT) || // Enable extended messaging
                        !workerant.setLibConfig(ANT_ReferenceLibrary.LibConfigFlags.MESG_OUT_INC_RSSI_0x40, ANT_PLUS_TIMEOUT) || // enable RSSI
                        !workerchn.assignChannel(ANT_ReferenceLibrary.ChannelType.BASE_Slave_Receive_0x00, 0, ANT_PLUS_TIMEOUT) || // Set to slave Rx channel
                        !workerchn.setChannelFreq(57, ANT_PLUS_TIMEOUT) || // Set channel freq.
                        !workerchn.setChannelID(0, false, 19, 0, ANT_PLUS_TIMEOUT) || // Set channel ID
                        !workerant.openRxScanMode(0) // Open channel in continuous scanning mode
                    )
                {
                    // If none of the above configuration worked, null everything out and throw an exception; keep looking
                    workerant = null;
                    workerchn = null;
                    throw new Exception();
                }
                // Add event handler to listen to the channels
                workerchn.channelResponse += new dChannelResponseHandler(workerchn_channelResponse);
                // Wait for ...5 minutes?
                workerant.openRxScanMode(5 * 60 * 1000);
As far as the message being flagged as 0x50 and that being invalid - good to know.  I'll try to enable channel ID in extended messaging and we'll see what comes up.
Thanks everyone