High humidity is a common cause but it sounds like your setup should help rule that out. If your temperatures are high that could throw off the rh a bit. Cigars will also go through secondary fermentation at times and give off that ammonia smell. Is this tupperdor closer to a source of heat than your other storage containers or maybe in direct sunlight? Just to help rule out rh levels do you run a hygrometer? Maybe run your heartfelt beads through a drying cycle. Do they change color when saturated? I always thought about getting some but never did.