You know (not targeting you OP). I always find it weird that people ask questions that no one can answer, sometimes.
It can be fun to spec. There are plenty of people who follow shows and enjoy them by doing things like keeping up with the actors' social media to find out what's being filmed and get a sense of what's going to happen.
OT: I don't think it's Round 1 of his fight with Kawaki. It's clearly a slash mark and Kawaki's weapon looks more like a staff-type weapon.
It's gonna be from doing something to protect Sarada while they're on a mission still as kids, and while Boruto's recovering from whatever Boruto protected her from, Mitsuki's gonna save them both.