IS this the plastic you talking about??
I have a couple of helmets where the plastic can be dented but the polystyrene remains fine because it is a few MM’s off the polystyrene. Apparently to help deflect some of the impact like a crumple zone. The dent shouldn’t compromise the preformance of the helmet as the plastic is only really there for cosmetic.
Similar to pushing hard onto the bonnet of a car. your going to put a dent in it.
Your first helmet sounds like it was a fault, the others, just you being awkward.
I also hope you spoke to the shop.