Is plastic saying "dishwasher safe" safe on both shelfs or only on the top shelf?
The reason some products are marked "Top Rack Only", is because many dishwashers have the heating element situated at the bottom of the interior, in close proximity to the bottom rack. It looks and acts much like the heating element in an electric oven.

Typically, this heater comes on during the final rinse cycle, to boost the water temperature. It also comes on during the dry cycle, to "bake" the dishes dry. Items placed on the bottom rack will therefore be exposed to a more intense heat than on the top rack, so things made of plastic can melt or deform.

Your dishwasher likely has options for "temperature boost" and "heated dry". If you turn both off, the heating element will probably not come on. Alternatively, try to place your container in the center of the bottom rack, in such a way that it's as far as possible from the heating element. If you use heated dry, take it out before that cycle begins, and/or check on it during the dry cycle