Harmless.
IANA is the governing body over the internet domain name system and IP addresses (among other things).
IP addresses starting with 169.254… are so-called “link local” (APIPA) addresses, which are auto-assigned and used by many devices in the absence of an DHCP server.
You seem to have at least one device in your local network which happily announces this APIPA address, even though it should not do this if it received a regular address per DHCP.
That’s how it gets into Plex trying to find out what kind of device it is (Plex is scanning the local network, trying to find routers and Plex clients) and ultimately into your log files.