Just so you are aware, the parts diagram for that tender is not listed at the moment. https://www.lionelsupport.com/...g-4-6-2-Pacific-8004
That said, knowing that other engines in that time era likely used similar parts - so an an example this 6-18003 https://www.lionelsupport.com/...nder-Only-6918003T01
So that particular series of 6 wheel truck
And then here is a topic on that series of truck and the coupler https://ogrforum.com/...x-wheel-tender-truck
These images from that topic- notice the plastic "thumbtack" style coupler armature- the part that moves and unlocks the knuckle for uncoupling.
I'm not sure if this is the exact one for that series of coupler?
https://www.lionelsupport.com/...LASTIC-W-THUMBTACK_2
That said, under load, it's not surprising for those plastic ones to uncouple since the pin can bend and thus unlock the knuckle. That's why either using a ziptie or o-ring, or rubber band, basically some method of further holding and locking that critical locking pin into the knuckle will better hold the load and not randomly uncouple.