Alright. It was a long time since I discovered such thing, I realised to try my SCORM package on the clear Moodle installation. And guess what? It worked fine.
After digging more, I found out that the issue is related to the course itself. When I created a new course on my old instance, applied the same package and passed it, course completion worked fine. So the solution is just to create a new course. That's it!