Why is this happening

Hello this model isn’t made by me but I had the same problem with my model so why does this happen. In my case this happened when I picked faces and turned on layer mask,that means when I paint, it will only paint on the area I turned the layer mask on. So the texture borders are being sharp eve though it mustn’t be like that. Do you have any fixes and have you ever seen this problem?
p.s. look on the picture