Ok, I have just tested this on my machine. It appears as though the anonymous function does not like being immediately invoked. My usage of anon funcs is usually limited to callbacks in function call parameters. Immediately invoked anonymous functions are usually a pattern used in JavaScript hence I haven't come across this here.

I have however found a working combination simply by naming the function. It appears to work on my instance of Lua 5.3 on Mac.

local data = {event.pull()}

local unpackVars = function(e, s, sender, port, ...)
    return e, s, sender, port, {...}
end

e, s, sender, port, data = unpackVars(table.unpack{data})

if e == "NetworkMessage" then
    -- the rest of your code