TYPO3 9.5.2 / Typoscript: sys_language_mode = content_fallback not working

Question

I added another language en to my homepage and added it also to the sites with entry point /en/. When I translate the startpage and switching the language to en, then it's working fine. But when I click at another page with no translation for en like /en/contact.html, then I got a 404 PageNotFound Exception. My ts config of the page:

page = PAGE
page {
    typeNum = 0
    config {
        absRefPrefix = /
        no_cache = 1
        noPageTitle = 1
        linkVars = L(0-10)
        language = de
        locale_all = German
        htmlTag_langKey = de-DE
        sys_language_uid = 0
        sys_language_mode = content_fallback
        sys_language_overlay = 1
}

[globalVar = GP:L=1]
page.config {
    sys_language_uid = 1
}
[global]

my sites config:

rootPageId: 2
base: 'http://some-domain.de/'
baseVariants: {  }
languages:
  -
    title: 'Meine tolle Webseite'
    enabled: true
    languageId: '0'
    base: /
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    navigationTitle: ''
    hreflang: ''
    direction: ''
    flag: de
  -
    title: 'Andere Sprache'
    enabled: true
    languageId: '1'
    base: /en/
    typo3Language: en
    locale: en_US.UTF-8
    iso-639-1: en
    navigationTitle: ''
    hreflang: ''
    direction: ''
    fallbackType: fallback
    fallbacks: '0'
    flag: en
errorHandling: {  }
routes: {  }
routeEnhancers:
  PageTypeSuffix:
    type: PageType
    default: .html
    map:
      .html: 0

Answer 1

A patch was finally merged: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60367 But after release of TYPO3 9.5.5. So expect it to be part of 9.5.6 then.