Hi Lawrence,
I have difficulty in believing this could have been a manufacturing defect. The hull is laid up in a mould and Dufour would have put continuous rovings lay-up across the bottom Any defect in that area would have been plainly obvious prior to inserting the liner.
Have you had the vessel from new and has it ever been grounded? Is there any damage to the liner and is the bond between the liner and the hull still intact?