This kind of thing is a hard lesson i learned many years ago, not all adhesives and stencil materials are created equal. I'm cringing reading the posts on this thread. Brings it all back to me. Ugh!
I'm sure it has gotten far worse since it happened to me 25 years ago. I bet many premade stencils are made overseas and who knows what they are using for adhesive and material?
And this is why I do not use any kind of stencil material unless I have prior experience with it. In fact, I'm so paranoid about this, I make all my own stencils now. For stripes, I'll get the measurements and tape them off myself. Yes its a little more work, but well worth it. And much of the time, it will some out better when you tape them off yourself, as not all sheetmetal is perfect, and sometimes you have to play around a little to get things even.
For non stripe stencils, before I started making them myself, I used a sign shop that used a stencil material I trusted.