[BUG] Heading type incorrectly transfers to the latter fragment when splitting a heading containing a line break

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• I have read the documentation and cannot find an answer.

Describe the bug

When a Heading node contains a LineBreakNode (inserted via Shift+Enter) and the user presses Enter to split the block, the heading type incorrectly transfers to the latter fragment. The original text loses its heading formatting and becomes a plain paragraph.

To Reproduce

1. Create a Heading (e.g., select "Heading 1" from the Block Type dropdown)
2. Type some text (e.g., first)
3. Press Shift+Enter to insert a line break inside the heading
4. Type more text (e.g., second)
5. Press Enter to split into a new block

Expected behavior

# first       ← heading type stays with the original text
second        ← new paragraph

Actual behavior

first         ← heading type is lost (downgraded to paragraph)
# second      ← heading type incorrectly transferred here

Screenshots

Before Enter (Shift+Enter line break inside H1):

After Enter (heading transferred to wrong fragment):

Desktop

• OS: macOS
• Browser: Chrome
• MDXEditor version: 3.52.4

Additional context

We discovered this while building a form-integrated MDXEditor setup (MDXEditor + react-hook-form) where we needed to ensure the markdown round-trip (markdown → editor → markdown) produces consistent results.

Our use case involves saving editor content as markdown to a database and reloading it later. Since the markdown ↔ WYSIWYG round-trip is inherently lossy (CommonMark collapses multiple blank lines, etc.), we normalize content at save time using the same mdast-util-from-markdown / mdast-util-to-markdown pipeline that MDXEditor uses internally — to keep the saved value and editor display in sync.

While testing all block types (h1–h3, paragraph, blockquote, lists) for Enter vs Shift+Enter behavior to verify visual consistency between the editor and rendered output, we noticed this heading split issue. It breaks the assumption that block-level formatting stays with the original content when a block is split.

This also means that a heading containing a Shift+Enter, when saved and reloaded through the mdast round-trip, can produce a different structure than what the user originally authored — compounding the round-trip normalization problem.

Start by reproducing the issue in MDXEditor 3.52.4 using an H1, Shift+Enter, more text, and Enter, then inspect the heading-splitting behavior. Done means the original fragment remains a heading, the latter fragment becomes a paragraph, and the behavior is consistent with the expected markdown round-trip.