So I learned in the issue tracker thread ( https://forum.vivaldi.net/post/749719 ) that it was a duplicate of "VB-28243 Remember sort state for mail folders" which indeed got fixed, but it was not a duplicate. The older bug was about the state of sorting (on the left in the screenshot), my bug was about the direction (ascending/descending, the arrow on the right) Screenshot_20240425_185332.png
The new Vivaldi 6.6 for Desktop introduces an improved Mail Search, allowing you to refine the search results, pretty much as described in this feature request. 🙂