Odd - here are my test flows
receiving end
[{"id":"191ad772.218719","type":"udp in","z":"3e6448b5.f21398","name":"","iface":"","port":"4447","ipv":"udp4","multicast":"true","group":"239.255.250.250","datatype":"buffer","x":190,"y":1060,"wires":[["77ae1bac.806f94"]]},{"id":"77ae1bac.806f94","type":"debug","z":"3e6448b5.f21398","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":390,"y":1060,"wires":[]}]
sending end
[{"id":"85716442.fb4208","type":"inject","z":"ab9333b5.f44f6","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":170,"y":2260,"wires":[["2b9912dd.4420de"]]},{"id":"2b9912dd.4420de","type":"udp out","z":"ab9333b5.f44f6","name":"","addr":"239.255.250.250","iface":"","port":"4447","ipv":"udp4","outport":"","base64":false,"multicast":"multi","x":380,"y":2260,"wires":[]}]
and yes
udp 0 0 0.0.0.0:4447 0.0.0.0:*
All working here of course... (two machines on same network segment etc etc)