Steps to reproduce the issue

1. Install Joomla 3.7.5 (last version before update to 3.8.0)
2. Create a template with two layouts for articles (lets call them a and b) and set this up in joomla
3. Set b as the default layout in Global configuration -> articles -> choose layout
4. Create articles and menu-items, do not create a menu-item for one article A1

Preliminary result: All articles use b as their layout, since it is the default option set in the component

5. Update to 3.8.0

Preliminary result: All articles still use b as their layout

7. Now create a menu-item for the article A1

Result: All articles use layout a as their layout, although b is set as default in the component

Expected result

Result: All articles use b as their layout, since it is the default option set in the component

Actual result

See above.

System information (as much as possible)

Joomla 3.8.0 upgraded from 3.7.5
PHP 7.1 / Apache
New URL routing is set to previous behaviour.

Additional comments

The problem was caused in my system, when the menu-item was set (which I forgot to do when the article was created long before the update). Issue is probably related to #17912.
Remedy: Set layout in every article explicitly
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