[FE][pageUnavailable_force] = true returns error code 500 instead of 503, skipping site handling
When setting the [FE][pageUnavailable_force] option to true, a default Typo3 error page is returned with a 500 error code header.
This happened with a freshly installed Typo3 10 with PHP 7.2, 7.3, or 7.4. Typo3 9 returned a 503 header as expected.
Setting this option also skips the site handling, so it is impossible to set a custom maintenance page, because the error is returned before site handling is executed. The default error page is returned even if a error handler for 0 is defined.
I don't know, if this is because of the 500 header or in addition to it.