Yeah thats really weird. They all talked about lines and being born with those but now he gets a line out of the blue.But its also talked about that it was unlocked in the past months / years but it now becomes visible? So either a timed transformation kinda thing or something else? That should also mean that Mash should be able to get a line at some point. It would also be hilarious that he cant get any because how strong his skin is or how fast he heals and a line cant form on him or something. Weird chapter overall imo.