Why do I have this brown line along top vermillion border & how to remove it? (Photos)

Have had lip filler 3 times in 8 years. Last time was about 3 years ago. I have recently noticed a brown line along the top lip vermillion border. It is very unsightly and I did not have it prior. I would like to know: Why I have it? What is it? Best treatment to remove it? Will more filler make it worse? If removed and filled again will it come back if removed? Will my lips be affected cosmetically if removal of line is done? Thank you