Are you pinging your Core?
It’s possible to never drop imcp but multicast could be dropped by a device and cause delays in the client finding the Core.
I’ve forgotten what Roon uses to discover and find within the subnet, MDNS? some kind of multicast I believe.
Here Danny says: We only use mDNS for Airplay discovery.
We also use multicast for our own discovery protocols (not entirely, but we augment with multicast).