AMP form method change from POST to GET

Question

I have a AMP page with contact form:

<form method="POST" action-xhr="mail/send.php" target="_top">

As you can see, the method is set to POST. But on my server side implementation (PHP), data from this form are sent via GET method.

echo $_SERVER['REQUEST_METHOD'];

GET

The form had previously worked without problems for several months. This changed few days ago out of nowhere (no previous code changes).

I have no idea what causes the form method change.

Reproduction Steps:

• Live URL: https://sigi-sitom.cz/#kontakt
• fill and send the contact form
• at the end of the error message is returned $_SERVER['REQUEST_METHOD'];