Not a Minecraft server about OC, but a Minecraft server running on OC. More specifically an 1.15.2 Minecraft server. (btw sorry for low video quality, but 2 Minecraft instances with one modded is CPU-intensive, so..)
You're welcome. I'm glad I could help:)
I was digging in the source code and it seems like a commit from January fixed that weird behavior. Some if-statement blocked incoming wireless messages while allowing wired messages to be accepted. In the newest version (18.104.22.168) it should work as intended.
I think I can tell you what to look for though:
It seemed really strange that with all 3 touching, communication worked fine. So I set up two computers: one with a normal network card the other with a wireless card tier 1, connected thru a relay with cables (relay without any card). If I send wired messages from the normal network card, they can be recieved by the wireless tier1 card, which they shouldn't. Maybe it's just a mix up that the tier1 card listens to the wired connection instead of the wireless one?