Nissan Navara plug

The previous owner of my 2007 Nissan Navara D40 STX 2.4-litre turbo diesel had it serviced by an independent mechanic, but since I bought it I have had it serviced by a Nissan dealer. At the 55,000 km service I was informed that the sump plug had been over tightened and the thread had pulled put, and an oversize plug had to be fitted. At the next service I was advised that the sump needed to be replaced at a cost of $470 plus fitting. The head mechanic at the dealership said that this has happened before on vehicles serviced outside the dealership, but when I asked the mechanic who had previously done the servicing about helping with the cost of replacing the sump that he had experienced the same problem with Navaras serviced at the Nissan Dealership. I was wondering that this may be a fault and perhaps should be covered by warranty. Have you heard of this before?