[BUG] Markdown headings disappear and formatting breaks with Japanese text output in v2.0.73

Description

When Claude outputs markdown with Japanese text, the markdown headings (##) disappear and formatting breaks. A period (。) appears floating alone in the middle of a line where a heading should have been.

Environment

• OS: Windows 11
• Claude Code version: 2.0.73
• Terminal: Windows Terminal
• Platform: Anthropic API

Steps to Reproduce

1. Ask Claude to explain something in Japanese that would generate markdown headings (##)
2. For example: "Version 2.0.73のリリースノートの解説をして" (Explain the release notes for version 2.0.73)
3. Observe the terminal output

Expected Behavior

Markdown headings should render properly with Japanese text, similar to how they work with English text.

Actual Behavior

• Markdown headings (##) completely disappear
• Punctuation marks like 。 appear floating alone on separate lines
• Overall formatting structure is broken

Regression

• Is this a regression?: Yes
• Last working version: 2.0.72

Additional Context

This issue started occurring specifically in v2.0.73. Possibly related to recent changes:

• v2.0.71: "New syntax highlighting engine for native build"
• v2.0.72: "Reduced terminal flickering"

<img width="960" height="1020" alt="Image" src="https://github.com/user-attachments/assets/56659470-2176-46bc-ad2e-1ba69b103100" />