Copying rich text only captures one heading at a time

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

I often copy the HTML-formatted text (from view or split view) into LibreOffice Writer to produce a .docx file. This worked fine for years, but in recent months I can highlight the whole text, but what gets pasted is only the first heading and any text following it up to the next heading. Possibly related to #387 but very different results.

#### Current behaviour

Provide steps to reproduce:

1. Go to 'hackmd.io/new'
2. Type or paste a Markdown document (often meeting minutes for me, lots of headings)
3. In the formatted view, select all the formatted text, copy with Ctrl-V
4. Paste into Writer
5. Only the first heading shows up

#### Expected behaviour

Entire formatted document should be pasted in.

#### Environment

I use HackMD on:

**Desktop**
- OS: Windows
- Browser: Firefox
- Browser Version: 136.0.4

#### Additional context

This had been working fine for years, but I noticed it had stopped working a couple of months ago.