Follow below steps to copy data from quote item to order item in Magento 2 through the below plugin

1. Create an di.xml in : /app/code/Vendor/ModuleName/etc

<type name="MagentoQuoteModelQuoteItemToOrderItem">
    <plugin name="custom_field_to_order_item" type="VendorModuleNamePluginQuoteCustomFieldToOrderItem"/>
</type>

2. Create an CustomFieldToOrderItem.php in : app/code/Vendor/ModuleName/Plugin/Quote

<?php

namespace VendorModuleNamePluginQuote;

class CustomFieldToOrderItem

{
	public function aroundConvert(
        MagentoQuoteModelQuoteItemToOrderItem $subject,
        Closure $proceed,
        MagentoQuoteModelQuoteItemAbstractItem $item,
        $additional = []
    ) {
        /** @var $orderItem Item */
        $orderItem = $proceed($item, $additional);
        $orderItem->setCustomField($item->getCustomField());
        return $orderItem;
      }

}