In "System Settings > Workspace Behaviour > General Behaviour", the "Clicking files or folders" setting is set to "Selects them (open by double-clicking)", and has been for years.

Since a recent update on Neon (the last two, I think) when I single-click a directory or file in Dolphin, it gets opened.
Folder view on Desktop still behaves as expected.

Is this just me or a general bug? I don't see it reported yet.

I've just tried on my KDE Neon system, which I last updated yesterday, and everything works as expected. I normally don't have the double-click setting on, so I turned in on after reading your post. It worked in Dolphin. I've turned it off again, which also worked.

I hope someone with more knowledge than me can help you find a solution to your problem.

Mehmet

I found the answer:

I'm using Kvantum as an application theme, and it has its own setting for this, in Kvantum manager.