If you can reliably reproduce the issue on command, and it sounds like this is the case, then it's the dealer's problem, not yours. Did they tell you to disconnect the battery during a scheduled service appointment specifically for this issue? If so, then that is totally acceptable. If this was either a phone call about the issue or just a "while I'm here" kinda thing, then make a proper service appointment and request that the tech comes out so you can demonstrate the issue.