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...