Collapsible List Format Is Corrupted

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• I have read CONTRIBUTING.md
• I have searched the existing issues and didn't find any that were similar
• I have considered creating a pull request with fixes instead of a bug report and want to proceed

Current Behavior

When you insert new content into an edited collapsible list, the original format is corrupted. You can only add new content at the end. Attachment： Figure 1 shows the edited collapsible list. Figure 2, content was inserted on the third line. I wanted the newly inserted content to be black, but by default, it automatically took on the same color as the surrounding text, which also overrode the previous settings.

Expected Behavior

Inserting new content does not disrupt the formatting that was previously set, The default color is black (the original default setting).

Steps To Reproduce

1. Press Shift+Enter in the collapsed list you’ve already edited.
2. Insert new content.
3. The original text’s color and bold formatting will become distorted.

Environment

- OS: win11 25H2\macos 27 beta7
- Version:0.56.9 alpha

Anything else?