I too from the headline assumed it was some sort of chemical contamination during manufacturing.
But then I read the words "The malfunction is due to multiple models of containers missing a pressure relief function in the center of the stopper."
How non-existent does your quality assurance have to be in order to miss such a critical, obvious and easy to identify flaw ?
Looking at the published photographs, you don't even need training to identify that manufacturing defect. A five year old could spot the difference between "lid has a hole" and "lid does not have a hole".
I would assume that it was some cost optimization that led to the removal of the pressure relief.
I've never seen a thermos bottle with a relief valve. Not sure if all manufacturers in Europe have nonexistent QA or if it's just not a thing that's commonly needed since you can't heat the contents up in there anyway (it's not like an insulated water boiler, which does have a release valve). Never considered that someone might leave food in there long enough to build up explosive pressure. Feels a bit like saying a missing cat detector function in microwave ovens points at them having no quality control