Scale and positioning of the print did not match the specified design

I'm an experienced designer and create all of my designs in Adobe Illustrator. I carefully lay out each design at the exact scale it should be printed, and I include a mockup for positioning directly within the file. That said, I’ve consistently run into issues with how the designs are scaled and positioned on the shirts. Even when I provide clearly defined dimensions and placement, the final prints often don’t match what I submitted. The sizing is frequently off, and the positioning can vary significantly from my intended layout. I brought this up with your support team during my last order and learned this detail: “For the artwork size, we have an automatic setting that resizes the artwork based on the ordered t-shirt size to better fill the available space.” While I understand how this is a good move for the average customer, as someone who eats, sleeps, and breathes design, it’s a constraint I wish I knew about up front during the submission process. I believe this could be remedied with a detailed template that clearly outlines positioning options and provides print area specifications that reflect how your auto-scaling system works. Know that I love your company and I'm sure that I'll place many more orders with you.