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I repeated following operations.
1. Type Japanese
2. Press return key (If not, the bug don't happen.)
3. Press command and space to switch an input method
4. Move the cursor
5. Type something

I think they are same as shown in my video.
I checked new version.
The bug didn't happen when I tried operations like on my video once.

However, if the operations are repeated, it still happen.

Thank you for improving.
Thank you.

I updated iOS today, but nothing changed.
It also happens on some text editor apps.
Here is a video.