I had the exact same issue and initially got the same responses as you are, that it was "cant error". To try to figure it out I actually ended up hanging targets from trees at varying distances and used the strings they were hanging from as plum bobs to line up with my vertical reticle so that I knew I was eliminating cant as a possibility and still had the same issue. I never did end up solving the issue but after much reading and searching and advice from the experienced folks here, I came to the conclusion that my scope rail was not properly lined up with the bore of the barrel. They were "cris crossed" so to speak. There were several that told me I needed to have my barrel indexed but I don't have the technical expertise to even begin to know where to start with that. I ended up selling the gun to a person who was fully aware of the issue. The closest I ever got to fixing it was shimming the scope rings at the rail so that the issue was less exaggerated. In my instance, I had to shim the front of the scope to the right so that it's point of impact was closer to the right and then also shimmed the rear mount further to the left so as to push the POI more to the right again.
Best of luck figuring it out and by all means if you ever do, please let me know how. It's an issue that I never solved and has been nagging me ever since.
Kind regards,
Cliff