@YounseoC 

update as this, don't use JSON encode

var GetEmailTemplateData = Class.create();
GetEmailTemplateData.prototype = Object.extendsObject(global.AbstractAjaxProcessor, {
    getEmailTemplate: function() {
        var templateId = this.getParameter('sysparm_u_email_template');
        var gr = new GlideRecord('sn_hr_core_email_content');
        gr.addQuery('sys_id', templateId);
        gr.query();
        if (gr.next()) {
            var templateData = {
                body: gr.message_html.toString(),
                subject: gr.subject.toString(),
            };
            gs.info('Script Include: body=' + templateData.body);
            gs.info('Script Include: subject=' + templateData.subject);
            return JSON.stringify(templateData);
        }
        gs.info('Script Include: Email template not found for ID=' + templateId);
        return '';
    },

    type: 'GetEmailTemplateData'
});

Client script: use g_form

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    if (isLoading || newValue === '') {
        return;
    }

    console.log('Triggering GlideAjax for sys_id: ' + newValue);
    var ga = new GlideAjax('GetEmailTemplateData');
    ga.addParam('sysparm_name', 'getEmailTemplate');
    ga.addParam('sysparm_u_email_template', newValue);
    ga.getXMLAnswer(function(response) {
        try {
            if (response) {
                console.log('Raw response text: ', responseText);

                var emailTemplate = JSON.parse(response);
                console.log('Parsed emailTemplate:', emailTemplate);

                g_form.setValue('u_email_body', emailTemplate.body || '');
                g_form.setValue('u_email_subject', emailTemplate.subject || '');

                console.log('client script works');
            } else {
                console.error('Error: No response received from Script Include');
            }
        } catch (e) {
            console.error('Error parsing response: ', e);
        }
    });
}

If my response helped please mark it correct and close the thread so that it benefits future readers.