Well, if it's annoying you that much, it sounds to me like you should attempt to warranty it. I don't have the tank, but I did have a tank where a somewhat similar system (hinged top) started out sort of loose. The problem actually corrected itself over time (the Ares) and I have no idea why but yes, mine was sort of sliding out of place from time to time. I don't believe I would have warrantied it, but if it hadn't improved I certainly wouldn't have bought a second Ares (which I did, the second one had no issues).
It may be a "small" matter, but it's not so "small' if it's preventing you from using the tank, so I would encourage you to warranty it. These things-- sometimes the slightest problem can just cause usability to plunge, so I would say, yes, warranty it. You have nothing to lose, especially if you like the tank otherwise.
Best of luck I hope you get it sorted out.
Anna