o is undefined / Repeated error "Field body is unknown" although the Gutenberg experience enabled node definitely has a 'body' field

Problem/Motivation

PHP message: Uncaught PHP Exception InvalidArgumentException: "Field body is unknown." at /var/www/html/web/core/lib/Drupal/Core/Entity/ContentEntityBase.php line 587

This happens just on loading the page in https://example.com/editor/reusable-blocks?langcode=en&context=edit&per_...

Does not seem to affect anything until "Add to reusable blocks" which triggers the same error in the console / PHP logs and a message in black in the ui, "o is undefined"

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes