This issue is now solved. Starting with Plasma 5.18, the desktop configuration must be set to "Desktop View" for virtual desktop switching to work anywhere on the desktop. If it is set to "Folder View", virtual desktop switching with the mouse wheel only works if the mouse is hovered over the pager on the panel. Researching on Arch forums, I found that this behavior is now considered "expected", even though a bug was filed with KDE. I will mark the issue as solved.