Incorrect config after update 9004

Problem/Motivation

Today I tested the upgrade to 8.x-2.6 and updating to schema version 9004 (form_mode_control_update_9004()). After the DB/config updates my form mode control settings no longer functioned.

During debugging I checked the /admin/structure/display-modes/form/config-form-modes page and found that all modes were set to "Default". So even though the config schema was updated it was obvious that something was wrong with the result. I manually set the correct form modes in the admin page and saved. The form modes began to work correctly. I exported the configuration a second time. There is a difference between the post-update config and the form-generated config:

Post-update config:

defaults:
  node:
    submission:
      edit:
        reviewer: submission_edit
        supervisor: submission_edit
        administrator: submission_edit

Form-generated config:

defaults:
  node:
    submission:
      update:
        reviewer: submission_edit
        supervisor: submission_edit
        administrator: submission_edit

If you haven't noticed, the update generated configuration with an "edit" key, whereas the form set an "update" key.

Steps to reproduce

I started with version 8.x-2.4.

1. Add a new edit form mode for nodes.
2. Move some fields around in that form mode for a content type. That way it's obvious which form mode is being displayed.
3. Set that form mode to be viewed by Administrators when editing the content.
4. Create an instance of the content.
5. Visit the edit form for the instance of the content that you just created. Verify that you're viewing the correct form mode.
6. Update the module to 8.x-2.6 and run database updates.
7. Visit the content edit form and see that you're not viewing the correct edit form mode.
8. View the site's active configuration (drush cex, etc.) and see that it has an "edit" key instead of "update".

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes