The query editor always opens with all query groups expanded. For larger numbers of queries, it would be convenient to be able to collapse all query groups at once. According to this idea: https://id...
Michiel,
The idea you mention in your post is referring to the Collapse all option in Fields panel of Report view, but not Query Editor.
In Query Editor, when you use Collapse all option in a group, it will collapse all queries under this group, which will not impact other groups. For your requirement, you can submit a new idea in Power BI ideas forum.
There is also a similar idea, you can vote it up.
https://ideas.powerbi.com/forums/265200-power-bi-ideas/suggestions/34865791-collapse-uncollapse-grouped-query-folders
Regards,
Lydia