JS for amp-form and amp-mustache missing on AMP page

Problem/Motivation

On a basic page I've implemented a form in the body field like the first example on: https://amp.dev/documentation/components/amp-form/.
The page is not valid because the necessary JS is not included. In the AMP Module in amp.libraries.yml i can see both of them. Why is it missing on my AMP page?

There are two different scenarios:

1. /admin/config/services/amp: Basic Page enabled; Manage display -> Field Body: Format „Default“

Steps to reproduce

1. Enable and configure AMP module.
2. Install AMP Theme and make a copy for a custom AMP subtheme of it.
3. Enable subtheme.
4. Set up a page with a form in body field like the first example on https://amp.dev/documentation/components/amp-form/.
5. View the page.

Result:

• Form is displayed
• Console: "The tag 'form' requires including the 'amp-form' extension JavaScript. (see https://amp.dev/documentation/components/amp-form)"
• Console: „The tag 'template' requires including the 'amp-mustache' extension JavaScript. (see https://amp.dev/documentation/components/amp-mustache)“

2. /admin/config/services/amp: Basic Page enabled; Manage display -> Field Body: Format „AMP Text“

Steps to reproduce

1. Enable and configure AMP module.
2. Install AMP Theme and make a copy for a custom AMP subtheme of it.
3. Enable subtheme.
4. Set up a page with a form in body field like the first example on https://amp.dev/documentation/components/amp-form/.
5. View the page.

Result:

• Form is not displayed (empty p tag).
• amp-form extension not loaded, but no error message in console
• amp-mustache extension is loaded, warning in console: „The extension 'amp-mustache' is referenced at version '0.1' which is a deprecated version. Please use a more recent version of this extension. This may become an error in the future. (see https://amp.dev/documentation/components/amp-mustache)“

Maybe helpful

• Adding amp-form extension manually to head is working, but Format „AMP Text“ still removes the form element from body field.
• Using a Custom block with the form in it on the basic page works, but amp-mustache is loaded with warning of deprecated version and amp-form is not loaded.