This is purely anecdotal, but I had a very similar experience in a company and I left. Life is too short to be that miserable. I had discussed my boredom and my general situation multiple times (both formally and informally) with my supervisor, as well as on one or two occasions with the department manager, and got nowhere; in fact, I was told multiple times that I needed to give it time to settle into the role (the fact that I was crushing it, as you seem to be, was irrelevant), and the company had the same 18-month policy. I was mostly blocked from taking on any kind of stretch project or anything above and beyond my description (which it sounds like you might not be). I ended up going above the manager's head after being stonewalled so often to negotiate my departure. He was not best pleased, to say the least, and I wish that situation had been less contentious, but I honestly felt and feel that I had tried to communicate the issue before then and gotten nowhere. There were some additional circumstances surrounding the initial job description, etc., that made the choice easier for me, but bottom line -- getting out of an organization that would intentionally hobble me like that, even after I had communicated it to them and proposed multiple alternative solutions, was definitely the right choice. Start looking around.