AMP form is not working correctly

I have incorrect work in AMP pages. Example.

After I click “Submit” button without any field entered (all is required) and privacy policy agreement, it shows:

“It appears your submission was successful.?<small>Even though the server responded OK, it is possible the submission was not processed. Please contact the developer of this form processor to improve this message.?Learn More</small>”

And the form is submitted correctly only if all required fields are filled with agreement, but I have the same status message in this case.