You actually might be right about a shadow, in a way. The black under the lettering was protected from light so even the slightest fading of the jersey would make the difference is visible. Just a guess.
Goof off 2.
Found at Home Depot and publix I think. Was it a white jersey? What color is the staining?
Have no idea what that first part was about. Black Majestic bought at a sports store at Wellington Mall. Not really a stain, just can still read the number and name of what was on it. More like a shadow.
About what I figured, even tho it hung on a rack for a year and can't see how it could get much light. I've washed it a few times and it still won't go away. It's not REAL noticeable, but it's there. I can live with it, I guess. It wasn't a fashion thing, it was to get a point across.
Try the goof off 2 or even the regular goof off from publix. Work it in with a tooth brush (one that is all white) and you'll be surprised. It's possible.
I took Hollandsworth 28 off a dodgers jersey and made it a piazza. The name was on since 96 or so and the glue was stubborn. A few hours later you couldn't see it.
That shadow may be a bit of glue.
I recommend goof off and then a washing with oxi clean or something similar