The yoast_seo module renders an entity in the frontend theme while in an admin theme context, in order to analyse the HTML output. This works most of the time, except if a template in the frontend theme uses the directory default variable.
The documentation of template_preprocess() says this about the output of _template_preprocess_default_variables():
variables that don't depend on hook and don't change during a single page request.
This is a wrong assumption. The default variables contain the directory variable, which depends on the active theme and that can change during a single page request.
Move setting the directory variable from _template_preprocess_default_variables() to template_preprocess(). This way it isn't included in the $drupal_static_fast variable and it can be reset from ThemeManager::render by unsetting the $variables['directory'] variable if it differs from $active_theme->getPath().
Needs work
11.0 🔥
Last updated
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
I added a proposed fix in the MR. This probably needs tests, so setting to Needs work.