KDE-6.3.4 / Dolphin-25.04.3 workaround:
Use KDE’s
systemsettings => Quick Settings => Clicking files or folders
to select the desired behaviour.
Then run this to respect the setting:
XDG_CURRENT_DESKTOP=KDE dolphin
Don’t ask why…
The above linked Single click always opens files despite system set to select. (dolphin) thread sounds looks like the SingleClick setting is handled by the KDE theme (?!!!). But the selected KDE theme is only applied if Qt thinks it’s running in a KDE session.
(looks like I’m not allowed to post links 
)