"I decided to just treat these boots with Meindl Sportswax. This is mainly because some members of this forum said that they had used other products (mainly Nikwax), which had possibly caused cracking of the leather which Meindl refused to cover under warranty, due to the use of products they do not recommend."
Graham Thompson ending up asking Meindl about care requirements on a simliar thread a few years ago.
As long as boots are cared for appropriaty - cleaned and re-proofed regulalry and occasional conditioner- then they would have a problem showing their goods as being 'fit for purpose' Using a ' you have to use our product or else' would not wash in a consumer complaint in my non ofcial opinion.
That said it does not sound like they are not performing. I would do as Parky suggests.
I find Nikwax is not as good as it used to be and now use Grangers on my old leather boots but have not noticed much difference really.