No clientside connection established

The typical client-side/server-side connection flow that you see to with HTTP for example does not occur with RADIUS and other MBLB traffic. I don't begin to know all the ins and out but I know it's very different. There's a great white paper by Lori McVittie here: https://f5.com/resources/white-papers/message-based-load-balancing

In the meantime, you might simply try saving the client info to a variable on the CLIENT_ACCEPTED event, then referencing the variable on the SERVER_CONNECTED event. I'm not sure if the mapping is accurate but.... Something like this perhaps:

when CLIENT_ACCEPTED {
    set client_info "[virtual] - [IP::client_addr]:[UDP::client_port] -> [IP::local_addr]:[UDP::local_port]"
}

when SERVER_CONNECTED {
    log local0.alert "$client_info ::: [IP::local_addr]:[UDP::local_port] -> [IP::server_addr]:[UDP::remote_port]"
}

You shouldn't have to worry about specifying the "clientside" or "serverside" contexts as the context is built into each event.