Hi @Raphael , that happened to me last year. Maybe your competitor is hidding his backlinks from ahrefs / majestic bots, so they can't show you his more powerful backlinks (usually PBN). By the other hand, I think Google cares a lot about statistics more than backlinks when you are on top, so let's say he has less backlinks, age and content, but for a reason people stay more time on his blog, visit more pages, do social actions, etc. So Google ranks him first. In this case you need to improve your metrics and try to get more and better backlinks from authoritative sites.
This year I failed miserably by trying to rank a parasite page with duplicate content following the logic that if me and my competitors we both have identical pages (shitty pages), I maybe could rank first. But there was a dude who was trying to do the same, and he won just because he did a 301 redirect 1 month before I redirect my 301...