I think it's the adhesive in the 505 that is sticking to your t-shirts--the knit is not as "slick" and even a surface as quilting cotton and it probably stuck better there.
I just used tissue paper to cover some crocheted doilies on a wall hanging so I didn't catch my hopping foot--I use taped and pin it down and it worked great. Don't think I would have used an adhesive.