TYPO3 cms-indexed-search core extension missing RouteEnhancer?

We are updating an ancient TYPO3 project, we already run on TYPO3 9.5 and it's looking good after a lot of refactoring.

Now I rework the ext:indexed_search section which got implemented into the T3Core (sysext:indexed_search). So if I disable/remove my site configuration, the search just works out of the box. Once I enable our default site configuration, holding:

base: 'ourDomain'
baseVariants:
  -
    base: 'ourDomain'
    condition: 'applicationContext == "Development"'
  -
    base: 'ourDomain'
    condition: 'applicationContext == "Testing"'
errorHandling: {  }
languages:
  -
    title: Deutsch
    enabled: true
    base: /
    typo3Language: de
    locale: de_DE
    iso-639-1: de
    navigationTitle: Deutsch
    hreflang: de-DE
    direction: ''
    flag: at
    languageId: '0'
rootPageId: 1
routes: {  }
routeEnhancers:

The search does not work properly anymore. I can navigate to the search page and still have the search form and stuff but no results are shown. I'm dumping $search inside SearchController and saw that it's actually empty no matter what.

After some research, I thought the RouteEnhancer configuration could be the problem, so I added some configuration to the site cfg

routeEnhancers:
  IndexedSearchPlugin:
    type: Extbase
    limitToPages:
      - 41
    extension: IndexedSearch
    plugin: Pi2
    routes:
      - routePath: '/search'
        _controller: 'Search::search'
    defaultController: 'Search::search'

So as child of our root-page we have a page called "Suche"(route=/suche) which holds the indexed_search plugin.

So the RouteEnhancer should work like domain.at/suche -> hit search -> /suche/search results, right? It seems like everything works but the data from the search input field is just missing... so there are no results.

I cannot get this to work for quite some time now, help is much appreciated.

a missing trailing slash in the form action was the problem. because without that slash a user would get redirected while loosing POST data.

solution:

.htaccess:

#redirect urls without slash
RewriteCond %{REQUEST_URI} /+[^\.]+$
RewriteRule ^(.+[^/])$ %{REQUEST_URI}/ [R=301,L]

site configuration:

routeEnhancers:
  IndexedSearchPlugin:
    type: Extbase
    limitToPages:
      - 41
    extension: IndexedSearch
    plugin: Pi2
    routes:
      - routePath: '/search'
        _controller: 'Search::search'
    defaultController: 'Search::search'
  PageTypeSuffix:
    type: PageType
    default: '/'
    index: ''
    map:
      '/': 0