OK, I understand what is happening.
At first I thought maybe it was the view you were using (Browser View) so I tried that and no difference. Then I rechecked your settings and noted you were using Play last and play next, and not Play Now as I mentioned in my steps above.
With all other settings left same, and under Player options default actions using Play next, or play last it works as advertised for a double-click, that track is inserted into or added to the existing list of now playing. This works the same as right click, play next etc.
However when I set the default actions to "play now" and double-click a track, the entire now playing list is replaced with the entire current contents of the main window. This is different from the right-click play now action.
but there does not seem to be any option setting to simply insert the selected (double-clicked) file to existing now playing list , and to play it now.
Using V2024 LATEST alpha or beta build on Windows 11, HP laptop, managing 13k tracks