This is a little backwards....the J armrest must be DOWN for takeoff and landing on 2 variants: B77W and A350. It is allowed to be UP or DOWN on one variant: A333.

i have no idea about other carriers policies. But the above is how CX FAs are supposed to enforce pretakeoff /prelanding.

IME, the confusion comes on the A333 when some FAs insist it be lowered. They are extrapolating from the B77W and A350 incorrectly, although I've found it's not worth the argument.

Lastly, on the A350 if the armrest won't go down and thus is broken in a mode that would leave the armrest up or disabled, the seat is not allowed to be flown. I know this from experience!