My lad has one of those A4s with that tender chassis problem - Merlin, in Green livery, in the Liliput box. Fortunately I removed the tender chassis from its body before any massive longitudinal stress occurred to break the tender ends. The example to which Esmedune linked has been photographed still in its box, so there may well be a broken tender body adjacent to the cab.
The tender chassis of my son's loco has continued to distort, such that it has a severe sideways bend. Fortunately (again) I had removed the motor, wheels and gears, and taken measurements of the chassis, before all that occurred.
Others of the Trix/Liliput A4s which have a different design of tender don't have that problem - my lad's Silver Link (in a Trix box) has a much better chassis design.