This implementation below worked!

 protected virtual void AMProdItem_InventoryID_FieldUpdated(PXCache cache, PXFieldUpdatedEventArgs e, PXFieldUpdated baseHandler)
        {
            baseHandler?.Invoke(cache, e);
            var row = (AMProdItem)e.Row;
            if (row?.InventoryID == null) return;

            var item = InventoryItem.PK.Find(Base, row.InventoryID);
            if (item == null) return;

           var itemExt = PXCache<InventoryItem>.GetExtension<INInventoryItemExt>(item);
            // Check if the custom checkbox (UsrAutoGenerateNumberChecker) is not ticked
            if (itemExt?.UsrAutoGenerateNumberChecker != true)
            {
        // If the checkbox is not checked, exit the method without making any changes
              return;
           }
            cache.SetValueExt<AMProdItem.preassignLotSerial>(row, true);
        }