Checkbox list rendering breaks without empty lines between items

## Description

When switching from markup mode to WYSIWYG mode, checkbox list items without empty lines between them completely disappear, except for the first item.

## Markup

```
[ ] foo
[ ] bar
[ ] baz
```

## Result

<img width="1086" height="293" alt="Image" src="https://github.com/user-attachments/assets/455a2a2a-427d-48cb-b9c1-ed959aa1cfab" />