Form Post strange behavior

I put in a Form in a post of mine. Basically, it submits data entered by user and uses it to draw some pictures. The form is submitted to its single post.

Now the strange thing is, if the user submitted from the homepage (https://www.hot-screensaver.com/), then it is OK.

But, if the user go into the post itself (https://www.hot-screensaver.com/2005/10/11/take-your-photo-online-for-free/) and submit the same form, the WordPress engine will throw “Sorry, no posts matched your criteria.”

Why? So strange…