No, it's a wrinkle - probably one I put there myself by stuffing everything in a bag to get it out of my way to clean. Or keep the cats off. Some bolts have the fabric wound funny and you will get a wrinkle here and there.
I'll try the vinegar. I usually do use the fabric.
I've had the same thing using cotton for shirts. I color block and embroider. I like all the wrinkles out before I embroider.
Does climate have anything to do with how fast or hard something wrinkles? It's dry here and in the winter, even drier in the house with forced air.