I've been using Dolphin for a while and currently I have two laptops with Arch Linux with Dolphin version 18.04.3, but the one with the issue has:
KDE Frameworks 5.48.0, while the one that works well has, 5.47.0.

So the issue itself is that thumbnail.so processes are constantly being created, even when doing horizontal scroll in split mode.
And no thumbnails are actually displayed.

I don't know if downgrading KDE Frameworks will resolve anything.

Or maybe this is a bug from this particular version of KDE Frameworks.

What do you recommend?

Thanks!

Are those processes using much shared memory or space elsewhere than in /tmp? What exact action on Dolphin causes this?

I noticed this too. And my KDE Frameworks version is 5.56.0.

In my case, every thumbnail.so instance takes around 15mb memory and 4mb shared mem. No other behaviour is perceived. A close observation indicates the thumbnail.so process is left running because of crash of Dolphin. And a new Dolphin process calls a new process of thumbnail.so, and so on.

Maybe it is this bug reported: Bug #404652