Arthur came back to life without any warning or whatsoever. He just one day popped up behind Merlin and took him by surprise, aka the merthur au no one needed
“Find an anchor.Something meaningful to you. Bind yourself to it, keep the human side in control.”
And that has a deeper meaning then just that. Merlin waits for Arthur even after roughly a thousand years. Like he’s the anchor waiting for Arthur to come back to him so he can update him on some stuff and just give Arthur the only piece of his Camelot, Albion, memories, that’s still alive.