"You cannot parse [HTML] using regular expressions. [...] Have you tried using [HT]ML instead with a parser? "™

(A more concise version can be found in the code snippet below)

function replaceTextBrWithDiv(html) {
  // Create an element that acts as a parser
  const parser = document.createElement('div');
  parser.innerHTML = html;

  // Modify an array-like when iterating over it may cause some issues.
  // Copy it first.
  const childNodes = [...parser.childNodes];

  // Index-based iterating
  for (let index = 0; index )
      index++;
    }
  }

  return parser.innerHTML;
}

try it:

console.config({ maximize: true });

function replaceTextBrWithDiv(html) {
  const parser = document.createElement('div');
  parser.innerHTML = html;

  parser.childNodes.forEach((node, index, nodes) => {
    const nextNode = nodes[index + 1];
    
    if (node instanceof Text && nextNode instanceof HTMLBRElement) {
      const div = document.createElement('div');
      div.appendChild(node);
      nextNode.replaceWith(div);
    }
  });
  
  return parser.innerHTML;
}

const content = 'text
text

sssccc

Here's my attempt before going for a (shorter) regex solution:

const dFrag = document.createDocumentFragment();
str.textContent.split('
').forEach(substr => {
  const div = document.createElement('div');
  let ul;
  if (!substr) {
    substr = '
';
  }
  div.innerHTML = substr;
  ul = div.querySelector('ul');
  if (ul) {
    dFrag.appendChild(ul);
  } else {
    dFrag.appendChild(div);
  }
});
str.innerHTML = '';
str.appendChild(dFrag);