OverriddenConfigurationTest::testOverriddenConfigurationRemoved ajax error

Problem/Motivation

There's an ajax error when the "Site branding" is saved in OverriddenConfigurationTest::testOverriddenConfigurationRemoved
Expected, the page doesn't reload.
Actual, page has an ajax error and redirects to /2, (stripping off the /user/ from /user/2).

Video:
https://www.drupal.org/files/issues/2019-12-26/ajax-error.mov →

Update, this also affects Drupal\Tests\settings_tray\FunctionalJavascript\ConfigAccessTest as well.

Proposed resolution

TBD

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet