What brand did you use? I use a polyester knit fusible interfacing on all of the T-shirts and Onesies I embroider and have never had a problem. I do use a Teflon sole plate on my iron for everything...It prevents scorching of any fabric and I always use steam when I apply the fusible knit.
I used a roll that I bought at my brother dealer. The directions didn't mention preshrinking at all
Cotton fabric doesn’t have instructions on the bolt to preshrink either but we learn : )
I've learned most of my lessons the hard way. I will get this figured out what works best for me.
Preshrink the tricot and the shirt separately before use. The tricot can be warm washed and dried in a low dryer or you can steam it with an iron without touching it. Do some practice pieces. Cut a small square or rectangle and measure it (try 3 inches). Then measure it again after preshrinking.