Presumably when you refer to the boot liner, you’re referring to the car’s actual variable boot floor that has the two height positions, rather than one of the flexible liners (VW or aftermarket) that can be bought and sits on top of the variable boot floor to keep the boot floor clean and catch liquid spills from items carried.
Boot floor - lower position
Boot floor - lower position
Flexible boot liner that sits on top of the boot floor (in lower position - versions are available for when boot floor is in the upper position)
Most of the flexible boot liners (as opposed to the actual variable boot floor) are manufactured from a very flexible material that would definitely sag if not supported by the variable boot floor from the underside. However, if you had the actual boot floor in the upper position, I suppose it is possible that it might sag or break if particularly heavy items are transported on it.
I suppose the lesson to be learned from your experience is that if heavy items are being carried, to put the variable boot floor in the lower of the two positions so it’s supported by the metal boot floor.
If you’re looking for a liner specifically for when the boot floor’s in the lower position, here’s one;
https://www.bootliners-uk.com/BLBT/volkswagen/golf/1670
