The mail form input error screen and confirmation screen are cached on the server side.
There is a problem with the content entered on a different PC being displayed.
The cache will be cleared in a few minutes.

The mail form is made by MW WP Form (ver. 4.4.0) plugin.
(The email form is embedded in a page with a shortcode.)
The server is WordPress.com, with caching plugins turned off.
I have a site built with the same plugin on another hosting service, and I have never had this problem.

I asked support but they could not resolve the issue.
Do you know anything about this problem?