Seems these lines in ProofreadPage appear in numerous stack traces with the same array_key_exists errors..
2) OutputPageTest::testHeadItemsParserOutput TypeError: array_key_exists(): Argument #2 ($array) must be of type array, null given /var/www/wiki/mediawiki/extensions/ProofreadPage/includes/Parser/TranslusionPagesModifier.php:88 /var/www/wiki/mediawiki/extensions/ProofreadPage/includes/Parser/TranslusionPagesModifier.php:59 /var/www/wiki/mediawiki/extensions/ProofreadPage/includes/ProofreadPage.php:261 /var/www/wiki/mediawiki/core/includes/HookContainer/HookContainer.php:338 /var/www/wiki/mediawiki/core/includes/HookContainer/HookContainer.php:137 /var/www/wiki/mediawiki/core/includes/HookContainer/HookRunner.php:2691 /var/www/wiki/mediawiki/core/includes/OutputPage.php:2151 /var/www/wiki/mediawiki/core/tests/phpunit/includes/OutputPageTest.php:379 /var/www/wiki/mediawiki/core/tests/phpunit/MediaWikiIntegrationTestCase.php:498 /var/www/wiki/mediawiki/core/tests/phpunit/phpunit.php:101 /var/www/wiki/mediawiki/core/tests/phpunit/phpunit.php:153