Wrong behavior when fragment applied on item with styled text in slide mode

The fragment on the item with styled text will have strange behavior, bug confirmed with @jackycute

Example:
```markdown
## What can students do?

- Bug report (as detail as possible)
- Bug reproduce (**confirm the reported bugs exist**)
- Test the new features (and you may find bugs!)
```

Start by reproducing the provided Markdown example in slide mode, focusing on fragments applied to list items containing styled text. Trace the slide-mode fragment rendering path and verify that each indexed item appears in the expected order without strange behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.