Hello
Currently on a freshly installed endeavouros with kde 5.22.1 / dolphin 21.04.2

I have already set the click option to double click option in the workspace behavior and found SingleClick=false in the configuration file, but dolphin did not seem to follow and is still opening file/folder in single click.
Desktop file/folder did not exhibit this behavior and i was able to select and open normally with double click.

--EDIT--
NVM i found the root of the problem.
For some reason kvantum theme is causing the problem.