KDE display 'global scale' has zero effect

I’ve been trying to mess around with the display scaling on my Manjaro install, and I’ve discovered that the ‘global scale’ slider in the settings does… absolutely nothing. I can set it on any setting and hit ‘apply’ and nothing changes - the display settings window flickers, but everything remains exactly the same size regardless of whether I set it to 100% or 300%. This remains the case regardless of which setting the ‘save displays’ properties’ option is set to - the only way to change the size of UI elements seems to be reducing the screen resolution.

How can I diagnose and fix whatever’s going on here?

Did you log out and log back in after changing the setting?

No; I assumed it would be an immediate change. Is that the issue? It seems inconvenient; it’d be nice to directly compare settings!