I am able to set 4 of the fields in the Tax Reporting sublist tab. Region of Origin. Delivery Terms. Nature of Transaction Code and Mode of Transport. I need to un set them based on the line on an invoice. I can unset all of them in the UI, however, my script won't unset two of the fields Delivery Terms and Mode of Transport. All the fields are part of the tax bundle and are select fields. My logs show empty After un setting them but the invoice manages to retain the values. Anything I can try to resolve this?

function beforeSubmit(context) {
    if (context.type === context.UserEventType.CREATE || context.type === context.UserEventType.EDIT) {
        const newRecord = context.newRecord;

        var hasInventoryItem = false;
        const lineCount = newRecord.getLineCount({sublistId: 'item'});
        for (let i = 0; i < lineCount; i++) {
            const itemType = newRecord.getSublistValue({
                sublistId: 'item',
                fieldId: 'itemtype',
                line: i
            });

            if (isInventoryItem(itemType)) {
                hasInventoryItem = true;
                break;
            }
        }

        if (!hasInventoryItem) {
            log.debug("No inventory items found, unset intrastat fields");

            const fieldsToUnset = [
                'custpage_country_of_origin_temp',
                'custbody_transaction_region',
                'custpage_delivery_terms',
                'custbody_notc',
                'custpage_mode_of_transport'
            ];
            fieldsToUnset.forEach(fieldId => {
                log.debug({title: `Before Unsetting ${fieldId}`, details: newRecord.getValue({fieldId: fieldId})});
            });

            // Unset the fields
            fieldsToUnset.forEach(fieldId => {
                newRecord.setValue({fieldId: fieldId, value: '' });
            });
            fieldsToUnset.forEach(fieldId => {
                log.debug({title: `After Unsetting ${fieldId}`, details: newRecord.getValue({fieldId: fieldId})});
            });
        }
    }
}

custpage_x fields ... aren't fields on the context.newRecord in your before submit

Oh right. So custpage fields are only accessible using Client Side script?

right, they're not real fields on the record, they're just temporary fields on the form in the UI

I can set them on the CS script, but I wanted to unset them in some scenarios in the UE script. So it sounds like I should be able to unset on the CS instead of the UE script.

you need to better understand how the fields are used by bundle. those fields could only be used for display purposes and the value stored elsewhere