I realized, that the UniqueSlugs addon never really worked, if more than one duplicates existed. Stupid mistake…
$entries = $app->module('collections')->count($name, [$slugName => $slug]);
The code above never counted the auto-generated slugs with numeric suffixes.
If you use this addon, you should update to the latest version, that now has support for localized slugs, too.
See this discussion for some more background #1086 and/or check the readme.