multiple text size issue in Emacs with CJK

[scturtle]

Describe the bug
While processing CJK characters in Emacs, some characters occasionally change to a smaller size unexpectedly.

To Reproduce
Steps to reproduce the behavior:

1. kitty --config NONE
2. emacs -Q
3. Insert 允许.
4. Delete the first space. 许 is shown in smaller size suddenly.

Screenshots
Before deleting the first space:

After deleteing the first space:

Environment details

kitty 0.42.1 (461e63049a) created by Kovid Goyal
Darwin J3QKDP979P 24.5.0 Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:49 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6000 arm64
ProductName:		macOS ProductVersion:		15.5 BuildVersion:		24F74
OpenGL: '4.1 Metal - 89.4' Detected version: 4.1
Frozen: True
Fonts:
  medium: Menlo-Regular: /System/Library/Fonts/Menlo.ttc
          Features: ()
    bold: Menlo-Bold: /System/Library/Fonts/Menlo.ttc
          Features: ()
  italic: Menlo-Italic: /System/Library/Fonts/Menlo.ttc
          Features: ()
      bi: Menlo-BoldItalic: /System/Library/Fonts/Menlo.ttc
          Features: ()
Paths:
  kitty: /Applications/kitty.app/Contents/MacOS/kitty
  base dir: /Applications/kitty.app/Contents/Resources/kitty
  extensions dir: /Applications/kitty.app/Contents/Resources/Python/lib/kitty-extensions
  system shell: /bin/bash
System color scheme: light. Applied color theme type: none

Config options different from defaults:
Changed shortcuts:
	cmd+k →  clear_terminal_and_scrollback
	ctrl+cmd+, →  reload_config
	ctrl+cmd+l →  clear_screen
	opt+cmd+k →  clear_scrollback
	opt+cmd+r →  reset_terminal
	shift+cmd+/ →  open_kitty_website

Important environment variables seen by the kitty process:
	PATH                                /Users/xxx/.local/bin:/Users/xxx/.cargo/bin:/usr/local/bin:/opt/homebrew/bin:/Applications/kitty.app/Contents/MacOS:/usr/bin:/bin:/usr/sbin:/sbin
	LANG                                en_US.UTF-8
	SHELL                               /bin/bash
	USER                                xxx
	LC_ALL                              en_US.UTF-8

Additional context

GNU Emacs 30.1.90
Development version e0c6f3e76568 on emacs-30 branch; build date 2025-05-22.