OnePlus - OnePlus Denial for repair of mobile under warranty

The phone screen started flickering. I went o the service centre and they denied to replace the screen for free, though the product is under warranty, as there was a tiny dent on the side panel. The dent was caused by the phone rubbing against the table. They are saying that the warranty is void though they are not able to establish a relationship between the dent and the screen flickering. The cost of the screen is too high for a middle-class family. It is therefore requested to kindly look into the matter and resolve the issue. Warranty slip of the product is attached herewith.