Trouble trying to automatically save checklist items which clicked

Solved by using Content-Type: application/x-www-form-urlencode with new URLSearchParams(body).toString() instead of JSON.stringify(body).

autosave-checklist.js

details.addEventListener('change', () => {
        fetch(autosaveChecklist.ajaxUrl + '?action=autosave_checklist', {
            method: 'POST',
            body: new URLSearchParams({
                action    : 'autosave_checklist',
                option: 'details-checkbox',
                item: details.checked
            }).toString(),
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
            },
        })
        .then(res => console.log(res))
        .catch(err => console.error(err));;
    });

    socials.addEventListener('change', () => {
        fetch(autosaveChecklist.ajaxUrl, {
            method: 'POST',
            body: new URLSearchParams({
                action    : 'autosave_checklist',
                option: 'socials-checkbox',
                item: socials.checked
            }).toString(),
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
            },
        })
        .then(res => console.log(res))
        .catch(err => console.error(err));
    });

    topicWheel.addEventListener('change', () => {
        // console.log('topicWheel: ' + topicWheel.checked)
        fetch(autosaveChecklist.ajaxUrl, {
            method: 'POST',
            body: new URLSearchParams({
                action    : 'autosave_checklist',
                option: 'topic-wheel-checkbox',
                item: topicWheel.checked
            }).toString(),
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
            },
        })
        .then(res => console.log(res))
        .catch(err => console.error(err));;
    });

    testimonials.addEventListener('change', () => {
        // console.log('testimonials: ' + testimonials.checked)
        fetch(autosaveChecklist.ajaxUrl, {
            method: 'POST',
            body: new URLSearchParams({
                action    : 'autosave_checklist',
                option: 'testimonials-checkbox',
                item: testimonials.checked
            }).toString(),
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
            },
        })
        .then(res => console.log(res))
        .catch(err => console.error(err));;
    });