That doesn’t quite look like a burn to my eyes. As well, having had no change in 3 weeks time doesn’t jibe with it being a burn or a bruise. Honestly, I’ve no idea what it is. The only thing I can think of is perhaps some sort of fungal or other type infection. Is it flat or raised? I think I might see a teensy bit of lifting at the scale at the farthest back, but hard to tell. Lifting is an indication of burned tissue. Does it have any difference in feel to it…like is it mushy, hard, etc? I have to advise a vet visit…not just to determine what it is but for treatment as well.