Do you experience this issue with other playback zones on your network? If the KEF LSX is the only device exhibiting this behavior, it would appear that your Roon installation and configuration are in good shape.
Have you updated your KEF LSX to the latest available firmware? Do they exhibit this behavior when connected using a wired network connection?
I use the KEF LSX every day in my Roon system without any problems. I can also use several zones via Airplay 2 (eg KEF LSX and Bluesound).
Works with me via LAN and WiFi.
For the LSX a new firmware came out. This should certainly be updated.
Have you tried factory resetting your LSX speakers yet? I have seen a few reports when factory resetting KEF speakers helped sort out some intermittent issues, it’s worth a try if the behavior is still the same since connecting them via Ethernet.