Dear Basj,
Thank you for giving me answer from AI.
I have my host OS installed in Japanese, so definately the OS in my Sandbox has
- the Japanese Language Pack installed
- System Locale set to Japanese
- Unicode Settings configured
And as it is Sandbox, it surly have all the System Update installed.
About the suggestion 4, are you sure Start 11 is a OLDER APPLICATION?
In fact I have all other programs run normally with correct Japanese language displayed. And as you can see in the screenshot, the winver dialog also shows correct Japanese texts.
I have a ChatGPT Pro subscription and the reason I ask questions here instead of asking GPT 5 Pro is that this problem is caused by your Start 11 program, not the OS or anything else.
I wanted to change the Sandbox OS into English to at least display your program but in Sandbox, the only display language avaliable is Japanese.

So please do not suggest non English users to test in Sandbox unless you fix this problem.
Back to the original bug, I reproduced it in my VM and here is the screen record.
There is a higher possibility to trigger it when you drag a tray icon out of or into the expand < panel.
BTW the keyboard shortcut I used to switch IME is Win+Space.
Hope this can help your engineers to fix it.