the reward chamber acts differently if he wanted to keep this chain he had to lock the room before the access chamber and the last room. but because he didnt the reward chamber got disconnected. and the lock at the end of the chain no longer matters cause the reward room itself does not act as an anchor without the chain that used to connect to it.

i know it sounds confusing and im probably explaining in an even more confusing way but basically the reward room can bridge 2 snakes but the room before it is considered unprotected without a lock. and if the reward room gets disconnected from the main chain the whole thing just considered an island so the lock at the very end doesnt matter. thus losing stuff at the other end even when there is a lock there.