Liquitainer™ Faltbare Wasserflasche - Product Reviews
My solution is to replace the caps with other caps which are readily available on other solid and single use water bottles. These replacement caps lack your pull up nozzle, which is also a less than adequate design, but at least they hold the seal and don't leak.
My advice is to review your design and supply chain for the caps as they lower the otherwise commendable design.
I did hesitate before breaking anything but that's how it's supposed to be done ...