I played an FFA as Malians and I eliminated a player and then killed their neutral monarch, but I never got the +50pop, I was pop-capped at 200 (yes i built more houses), and the +50 pop notification never triggered either.
I had archers with poison arrow, so I suspect that is the reason that the monarch kill didn’t register correctly. I cannot think of any other reason why the kill of the monarch would not be attributed to me.