I couldn't find anything about this issue anywhere so I'm concerned it is a defect with only mine. My brand new T bag and luggage block is somewhat loose fitting and bounces around in the mount quite a bit over bumps.
It mounts and clicks into place quite well, but there is plenty of up and down movement even when it's clicked into place. There isn't so much left to right movement, but nontheless, I'm finding it quite annoying. Most Brompton luggage reviews claim the system is tight fitting with no rattling but mine is rattling plenty.