Updater ignores additional config.php files

As it is described in https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/config_sample_php_parameters.html#multiple-config-php-file, it is possible to use multiple config.php files.

Unfortunately, however, the updater.phar file seems to ignore the other config files, which causes the updater to not work properly on some otherwise smoothly running Nextcloud instances.

One problem I and many other Nextcloud users have as a result is the following:
There are hosting plans where the basic file path is different when a user accesses Nextcloud via web browser than when, for example, OCC and updater.phar are accessed via SSH. Then a dynamic adjustment of the datadirectory is necessary. This can not be done in the config.php, because this could be overwritten by Nextcloud at any time again to a fixed path when updates are applied or the admin changes other values in the settings and hence the main config file gets rebuilt. Therefore, many fix this by creating a separate config file, like this one at https://github.com/froonix/webhosting-mods/blob/master/httpdocs/nextcloud/nextcloud/config/data.config.php to overwrite the value of the config. This works fine except for the updater.

Users of such and other usecases probably wonder why occ works fine, but the updater.phar has problems and may not get the idea that multiple config files are supported for Nextcloud, but not for the updater. If you have read the documentation you don't expect such a behavior and I could imagine that this is responsible for some bug reports.

Start by tracing how updater.phar loads configuration and compare that path with OCC, using the documented multiple config.php behavior as the expected reference. Reproduce an instance with an additional config file that overrides datadirectory, then verify that the updater recognizes the same configuration and completes its update correctly.