Hi Viktor, well not really but this workaround is at least allowing to send music to Moode player (https://moodeaudio.org) for the time being. Wish I could have identified the specific root cause, but with all my testing I could not go any further.
At some point I gave each AP (including Omnia) a different SSID and only when connected to the Omnia WIFI, I could not see the shairport resource.
Also tried to disable multicast snooping but no luck ([SOLVED] mDNS/Avahi/Zeroconf on bridges (e.g. br-lan)).