While trying to create a PR for #6656 I searched an hour for a bug in my code until I installed a new staging and saw that "I'm not guilty" ;-)

Test:

• Current staging.
• Extensions > Module Manager
• Select "Site" as Client to see frontend modules.
• Select Position "atomic-bottomleft".
• Select Language: "All"
• Select now "Administrator" as Client to see backend modules.
• Message "There are no modules matching your query"
• See Position filter field: "- Select Position-". That's confusing.
• See Language filter field: Still "All"
• Click cross next to Search filter to reload page. All OK afterwards.