Thanks for your reply.
I tried some more and it seems to be not random after all.
The prompt doesn't appear when you are in the main menu/home page (described by contentObjects.json). EDIT: OK it's normal. I've misunderstood how the bookmarks works and I though you had the ability to continue or restart the course. But in fact it's "go the bookmarked page or whatever" or "stay in the home page". Sorry for this.
Also, and this appears really random, sometimes when I re-open a course from the lms, I have a blank screen with Uncaught SyntaxError: Unexpected end of input in my Chrome console. But as soon as I close the course and re-open it again, everything works...