TYPO3 Core

If you create tt_contents with language set to "All" aka 1 then those records are only shown in foreign languages. In the default language (sys_language_uid = 0) they are not shown.
It seems that in ContentObjectRenderer->getQueryConstraints the constraint "AND (tt_content.sys_language_uid in (0,-1))" is only added, if the same page is opened in other languages. In the default language it's always "AND (tt_content.sys_language_uid = 0)". In TYPO3 8.5 the expected constraint is also added in the default language.
The problem is, that in ContentObjectRenderer>getQueryConstraints the LanguageAspect's doOverlays() method is queried, and this only return true if sys_language_uid is greater 0.
For me the solution is to change the else part on line 6936 (TYPO3 9.5.1) in ContentObjectRenderer.php from

$languageQuery = $expressionBuilder->eq($languageField, $languageAspect->getContentId());

to

$languageQuery = $expressionBuilder->in($languageField, [$languageAspect->getContentId(), -1]);

but I don't know if there are any unexpected side effects.

Side note: I also made some other tests and for me extbase records and even the RECORD TypoScript object works as expected. I think only CONTENT TypoScript object and code parts which depends on ContentObjectRenderer->getQueryConstraints are affected.