Sounds to me like a network issue.
First guess: a switch with broadcast storm protection enabled.
As soon as network broadcast traffic reaches a certain level the switch will prevent any further broadcast transmission for a certain time.
This must be switched off for use with artnet.
It can also be an artnet interface issue. A lot of the cheaper solutions don´t have enough memory to work when sending a lot of artnet universes. With broadcasts the device has to listen to all information sent...
Olli
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Oliver Ranft, Aachen, Germany