It's not clear what triggered the change in behaviour, but currently when I click on a virtual desktop in the KDE/Plasma pager, KDE no longer switches to it (well, some of the time)
If I click about, it seems to send a window from the current desktop to that other window, or sometimes brings a window from that virtual desktop to the current one.
In order to switch between desktops I have to use the mouse's scroll wheel whilst hovering over the pager.
Occasionally, clicking on the pager behaves as it should and switches to that other desktop.
When the issue's present there just doesn't seem to be any consistency in the behaviour at all. Didn't find anything searching online for kde pager bug clicking moves window