I read somewhere that the cabin in the B777 was a bit heavy compared to what they were expecting. Not sure if this is true with the a350 as well.
So that might be the reason it was pulled off a few long hauls.
It does seem that the a350 is a much better fit with CI as apart from LAX most other long haul routes don't really need the extra capacity of the B777. JFK could go A350 if they are struggling to make it profitable. Filling the plane maybe but very low yield to do it.