Two days ago I installed KDE on my Linux Mint, which had Cinnamon by default. It was mostly working fine, but I had this weird bug. Though in the settings (Workspace Behaviour) I had it set to open folders and files with double-click, that only worked on my desktop. Within the actual file manager (Dolphin), it still opened things with a single click. So I went first to the KDE subreddit, and somebody told me that it's caused by using KDE on Mint, as Mint doesn't support KDE. So I thought "fine" and installed the latest Kubuntu LTS, with a complete erasure of my HDD.

So right now I'm having a clean fresh install. Within the first couple of hours I didn't have that issue. Then I folded my laptop and left it like that for an hour, so I assume it was in the sleep mode. When I returned and opened it again, I went to Dolphin and the issue was present again. I don't know the exact moment that it returned though, since I wasn't checking Dolphin every few minutes to see. But as far as I can tell, I didn't really do anything out of ordinary to cause it.

EDIT: Feeling like an idiot. One of the first things I had done after installing the system was changing the theme (the theme in question was one of the key factors in me moving to KDE), and turns out it was a kvantum setting by default to open things with a single click. Which is the reason I didn't notice that initially, as I didn't move onto another theme right away, but only within a couple of hours.