Sorry - It's a fact.
I am not saying that all of the factories do it that way - I agree that there are some extremely competent and ethical ones (including yours).

I agree that very few designers would agree to selling their designs to multiple customers. The issue is from the factories that are contracted to produce for particular companies.

Allow me to explain.

Company "X" asks manufacturer "Y" to manufacture say 200 of a particular design. Because of internal rejection rates, Manufacturer "Y" actually makes 230 planes but presents the best 200 of them to Company X. Company "X" then accepts say 195 of them and pays for them. So far so good - Company "X" has their planes (with an apparently low rejection rate which is within their agreed terms). Meanwhile manufacturer "Y" has 35 planes that they have not received any income from. Those planes are often put on the market as some other brand - or are sold on an auction site as a "non branded" item. They are rarely presented as the original item. It is not the designer that has done this - it is the manufacturer.

At no point has the designer agreed to sell his design to another customer - those sales often take place without his knowledge. Meanwhile there are claims of "copying" of designs being made.

I have seen it happen. It is not just a vision.

I know of your companies products and I agree that they are one of the most ethical companies to deal with - unfortunately not all manufacturers are as ethical. I have experienced the other side of that first hand.