Input broken when saving changes with markdown enabled

Problem/Motivation

When I edit content in markdown mode on an existing content and save changes, the displayed page (or any other content type) looks fine, but when I edit it once again, the input in the field that uses markdown is broken (attached screenshot).
Now, when trying to save changes when in markdown mode, nothing is being saved, to properly save the content, one has to always switch off markdown before saving.

Steps to reproduce

* Create any content with a text field with markdown
* When in markdown mode changes are not saved at all (field is empty)

* Edit the content after saving with markdown switched off,
* switch markdown on, save, edit again
* The content will be broken as in the attached screenshot

Note that

$node = \Drupal::entityTypeManager()->getStorage('node')->load([node_id]);
$node->body->first()->value;

displays the correct value after the first save (when the edit page already displays a broken value). After the second save, the value is broken in db as well.