What's the issue?

When a form containing a textarea and input field is cleared, only the input field will reset to its binding value.

Steps to reproduce:
https://output.jsbin.com/rubuhexagi

1. Click Open lightbox
2. Click Add !
3. Click Clear form and close lightbox
4. Click Open lightbox

Actual result:
Only the input field retains the text "hello!" while the textarea field remains blank

Expected result:
Both the input field and textarea field retain the text "hello!"

What browsers are affected?

Tested with Chrome

Which AMP version is affected?

Tested with Version 1902151859190