JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li data-id="122">122<input class="sort_index"></li>
  <li data-id="124">124<input class="sort_index"></li>
  <li data-id="132">132<input class="sort_index"></li>
</ul>

JavaScript

const sortPosition = '124|132|122'


function syncIndexes(positions) {
  const lis = [...document.querySelectorAll('ul > li')]
  
  positions.split('|').forEach((pos, i) => {
    lis.find(li => li.dataset.id == pos).querySelector('input').value = i
  })
}

syncIndexes(sortPosition)