Sitemap tree problem when using multisite setup

[klickparkdominik]

Affected Version of Concrete CMS

9.x

Description

We are not able to select a sitemap tree via dropdown when using a multisite & multilanguage setup.

We are unable to select any language tree for the second site.
We are able to select the default sitemap tree for all languages.

First two trees working, last two not working.

In addition there are multiple flags shown in the dropdown.

3 Flags visible?

Using v9.0.2 with PHP7.4.

Forum users are also reporting this problem:
https://forums.concretecms.org/t/multipage-site-setup-issue/2341/2

How to reproduce

• Install Version 9.0.2
• Add multi language configuration for DE & EN
• Add multi site configuration with a second domain
• Try to select DE or EN language in the sitemap selector dropdown for the second domain
• Nothing happens, the default sitemap tree is still visible

Possible Solution

When logging out from https://#main-domain#/login and logging in via https://#second-domain#/login the second sitemap tree is visible by default and working. But as soon as we switch the selection, we are unable to go back.

Sitemap shown but not selected?

Workaround

Removing the following lines will make all languages accessible and fixes the problem.
But the nice groups in the dropdown get lost.

https://github.com/concrete5/concrete5/blob/579e9f6339944de9f0134d59402d7fd7411f05c3/concrete/src/Application/UserInterface/Sitemap/StandardSitemapProvider.php#L104-L108

Additional Context

The problem seems to be similar to this:
#10152

And maybe this:
#10292 (comment)

[aembler]

This has been fixed in develop and will be released in 9.0.3