YES, king sized beds NEVER have enough support down the middle.
Year one had a couple bust the bed and call from their room at 2am. They were so embarrassed, and you all know "the bucket story" from there.
Our own bed has added supports underneath the center from the day we bought it as it sags - even with a new mattress - it is a heavy duty log bed made in Idaho BUT the supports are not strong, just the rest is strong, and it cannot handle the weight of the mattress, let alone a couple. When our 12 lb cat jumps on it IT MAKES NOISE!!!.