It's not unusual for the jaws not to unlatch if there is still some strain on them. I consider that a safety factor.
๐ You just need to play with the truck position relative to the trailer and weight still on the hitch/truck.
I can see that the lever under the fender could be loose when there is weight on the hitch. When you tighten the pull down bolt, that puts some upward force on the ball and keeps the lever tight. However when you have your 5th wheel loaded it's very possible that the weight will allow the ball to move enough to take the upward force off the lever pin. Now you could pull the lever if you were so inclined.
I've never tried, but I can see how it could work like you're describing.
Howard and Peggy
"Don't Panic"