I found the cause!!
For example, when we access
https://www.hkdyukkuri.space/index (my private website in Japanese), it shows the top page saved as index.html . In short, the URL /index is accessable as well as /index.html .
Maybe that function causes the issue.
Now I am contacting the company hosting the server to ask how to invalidate the function. This problem will soon be solved.